I have just become aware of some design considerations you may wish to consider when ordering hold pickup locker units. I learned from our Mobile Services (bookmobile) Manager that due to human physical limitations, they are avoiding use of the uppermost and especially the lowermost lockers when loading our standalone outdoor unit. As you can see from the photo, there is plenty of room for another column of lockers on the right side and our library may in time add that. It remains to be seen how the lowermost lockers will fare in snowdrifts and ice storms, and in retrospect it probably would have been preferable to place the locker unit on a low, sturdy, enclosed stand with the lockers configured 3 across by 10 down.
Not to beat this poor thread to death 
with posts, but this query was written specifically to diagnose a problem with holds in these lockers.