Take a look at the age mix of kids you have. If there aren't older kids to show them how to use their imagination then it will be awhile before it happens. Also as you move your toys take a look at each one and decide what the age level is for that item. If you don't have kids that age put it aside. Sometimes the one puzzle with the big pieces gets lost on the shelf of puzzles and so they either dump all the others and can't put the pieces back or don't go there at all cause when they do they get told not to dump - which is a normal toddler activity.
For your group learning times why not teach then how to use the toys instead of a lot of what you are doing now. Show them how a toy works or how to expand the use of other toys like putting blocks in the dump truck and dumping them out, lining up the cars in a row, sorting the blocks by colour. Then hopefully they will start to do some of those things in play.
I agree that it does sound like the majority of the day they are told what to do and how to do it and are "entertained" so it isn't a surprise that they have not learned to play alone. At home parents are doing the entertaining or giving them something to do such as turning tv on for them so there is just this awkward hour they need to get through. At the same time having some down time and doing nothing isn't a bad thing for them either. Mostly I would make sure there are options available that are age appropriate but not so much that it overwhelms and then just let them be - they will figure it out. Once you do move down to the other room I am guessing it is a bigger space - spreading out activities makes a difference too so they specifically go to a place to do that activity not stand and stare at a shelf of everything.
Rubbermaid buckets of toys out of rotation make great play surfaces to just put a toy on. Then trade out a few every few days. Toddlers wander and carry things from place to place - that is normal play for them.