What agitation method do you use? My experience was with inversion agitation in a Jobo 4x5 system and a half-full tank. You said you used 35mm, which is commonly agitated with inversion, typically every 30 seconds.(posted 8507 days ago)Perhaps if you described your processing technique better we could give better guesses at the cause of the problem. What kind of tank, pour developer through lid or put reels in tank already filled, agitation method and frequency, etc.
Caking in different chemicals can be different results: moisture can cause clumping, but no degradation, for example with simple salts. In others it can be the result of a chemical reaction with air or moisture, which will degrade the chemical.