The most likely culprit - the inks are improperly cured! Over-cured or under-cured ink can cause your print to wash out/crack.
Plastisol ink needs to achieve at least 320 degrees to be properly cured. That is 320 from surface to garment, so if you have a thicker ink like a white you will want to give it a little more time to penetrate the garment fully. Once the ink hits 320 degrees, you will want to let it dwell at that heat for 10-20 seconds. Always do a wash test on the very first shirt you print!
Water based inks should be cured for a minimum of 90 seconds at 360°F. Ideally, printed fabric should be cured for three minutes at 300-320°F. In case you can't achieve these curing specs with your dryer, you can add Warp Drive to your ink. Warp drive allows the mixed ink to cure at a very low point. The mixed ink typically dries at around 180 ºF but can be cured at higher temperatures of up to 280 ºF. Ensure that the ink is left to set for at least 48 hours before washing.