The most obvious will be that the medium is losing too much moisture and drying out. This can happen in vented flasks as moisture slowly will escape through the vent. Contamination (mold) is also pretty obvious to spot. In that case you would deflask and compot the plants instead of reflasking anyway. The other issue, if the plants are too crowded, would be the buildup of "phenols" -waste products- which would be evidenced by changes in root growth(dying roots).Be careful not to leave the flasks in direct sun or very bright light, heat will build up and can kill the plants quickly (everything turns brown).
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