Method 1

Not assemble your development tank correctly.

This is on me. I'm sure it won't be the last time I do this wrong. I'm using the Lab box daylight development tank to develop my 35 and 120 film. It requires to be assembled use and dissasembled for cleaning. I was in a rush and wanted to get my photos as quickly as possible, and missed an entire piece of the machine. The film feed. Therefore my film did not actually wind correctly on to the reel. The results was most of the film was a blurry mess, totally unusable.

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Method 2

Your developer has expired

This is some scary stuff, you could do everything correctly, assemble, load, timing, changing chemicals. And then when you go to open the tank and see your photos for the first time to rinse. You get what every home film developer dreads. The entire roll is blank.

The primarly reason for this is that your development chemicals have expired. Meaning that they no longer have the chemical properties to react with your film and separate the light, dark, color layers, etc. Then the blix (Bleach/Fixer) comes in and wipes everything else off, leaving you with a roll of .. nothing. Alternatively, there are other reasons where your roll is *mostly* blank, except for the manufacture data pre-exposed on your roll, this would mean that your chemicals are still working...but maybe your camera or camera settings might be the cause.

Here's someone much more qualified than I am to explain in detail what happened and what to do about it. Why is my roll of film blank an Youtube video by the folk at Cinestill.

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Method 3

Time, temperature, agitation, and order of operations

Another way to ruin film development is the actual development process itself. This is a chemical reaction, and in chemcial reactions, there are very specific conditions that need to be met, or your film will come not how you expect. Specifically, differnt film stock at different film speeds in different development chemicals will all require the correct timing, tempurature, and amount of agitation. Make sure to check the film manufacture's website / product info to see if there is anything that you need to be aware of. Similiarly check to make sure you understand the instructions for your development chemicals, so that you are know what are the suggested ratios, dillutions, agitation timing, and most importantly order of operations.

There are many different ways that this chemical reaction can go wrong, I shall list out some principle problems and likely causes.

Problem	Cause	Notes
Photo comes out too light	The temperature was too warm or you let the development phase take too long	Make sure to use a themometer and timer to accurately develop
Photo comes out too dark	The temperature was too cold or you completed the development phase too quickly	Make sure to use a themometer and timer to accurately develop
Photo comes out blank	See above re:expired developer, but the other reason that this is possible is mixing up the order in multi-step development.	Make sure not to contaminate your developer with any bleach or blix!
Photo has water spots/bubbles on the image	During agitation phase make sure that to tap the bottom of the tank once in a while to dislodge any air bubbles	Also to fully dry out the film and hang it straight as possible to prevent warping

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Bonus method

Not handling your film carefully after development

Here's a bonus thing to watch out for, and I do this all the time. When removing the film from the reel, hanging the film up to dry, or cutting the film for scanning make sure to handle the film with care. I usually end up with a lot of scratches and dust on my film by the time I scan them. I should be using gloves to handle film, lint free paper to wipe clean, and clean surfaces if I have to rest the film down. But I can be lazy sometimes and not do any of these things, but I'd like to think I'm adding *character* to my images with roughness, but usually I just end up spending a lot of time in Photoshop removing scratches and dust.

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In conclusion

The moral of the story is, to always take your time preparing your film for development. Whether it is assembling your development tank, loading the film, making the chemicals do what you want, and also handling the film itself once you finally do everything correctly. The end results should be photos that are well developed even if they are not well composed. The development and scanning process is very complicated, but I'd like to think that it's half the fun of taking photos with film.

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Sources

Why is my roll of film blank an Youtube video by the folk at Cinestill.
How to preform a snip test Watch this to learn how to check if your developer is expired
How to load film on to a reel This is a basics video, but make sure you are super familiar with your tanks/reels