started too fat - started at about 15 or 16% last time. above 12 or 14%, you'll likely gain more fat than muscle. starting at lower bodyfat percentages, i have a better chance of gaining more muscle vs fat. at that time, i should have done a maintenance month instead.
did not slowly add calories - by going from a 500 caloric reduction to a 500 surplus, my body could not keep up, so it switched to storing fat.
too much protein - at that time, my percentages were 20% fat, 40% protein, 40% carbs. so i would get more than 1.5x of protein grams per bodyweight in lbs. thats more protein than i needed.
not enough carbs - needed the extra carbs for energy and recovery.
too much sugar - from fruits, dairy, and a sugary post workout drink. going to cut out the sugar substitute products too.
didnt train heavy enough - tried lifting 10-12 reps when i had more calories. instead i should have been lifting 5-8 reps, and saved the higher reps when on a caloric deficit.
way too much sodium - was probably getting about 7.5 grams, or 3x what i need.
not enough water - was getting too much caffeine at the time, and that was dehydrating me.