I have a Samsung Galaxy S2 and when I tried to download applications from Playstore it showed a message "App will be installed soon" but that never happened. I think it was because there wasn't enough main storage. Device Manager/Storage showed Device Memory as Total Space 1.97GB Available 190MB and USB Storage as 11.50GB and available and available 10.69GB. So there's plenty of storage overall but it wasn't being used/managed effectively by the phone.
After much searching on this and other forums without any luck I think I've finally found the solution myself. If you go to the main Settings menu and then Device and click on Storage there is a Format USB Storage option. I did this even though it warns it will erase any files held in that storage area as I was less concerned about that than the fact the storage didn't seem to be being referenced at all by the S2 device. Anyway, once done all applications are now downloading fine from Playstore and are being stored in the USB Storage area.
I suspect that the problem with accessing the USB Storage may have arisen after I upgraded the Android version on the S2 from Ice Cream Sandwich to Jelly Bean v 4.1.2 and that the reformatting of the USB Storage area has now recreated the right linkages with 4.1.2 - can't be sure but this may be the reason. Anyway, it's fixed now.