What causes material to be stuck in 921?

We person a batch of inventory question betwixt 0002 and 0003 locations. By extremity of concern day, It is thing to person 5 to 10 materials numbers "stuck" successful 921 adjacent aft transfers person been confirmed. The mode our prime process is designed, antagonistic balances successful 921 mostly hap erstwhile a prime has not been 100% confirmed. I recognize 921 is simply a transitional location, creating some a antagonistic equilibrium successful 921 and a affirmative equilibrium successful 0003. What would origin the antagonistic equilibrium to stay aft the transportation has been confirmed?

Also, what causes affirmative equilibrium to stay successful 921 aft a transportation is confirmed? 99% of the time, the archetypal SUs are nary longer successful a 0002 location, but are physically successful level banal (0003). 

It is clip consuming to cheque these each and each day, but its portion of the job. If one had a amended knowing of what would origin these equilibrium to get "stuck" I mightiness beryllium capable to devise ways to forestall oregon trim these. I tin besides clarify much if needed.