Redbox has certainly become more popular as their kiosks have multiplied, but it seems possible that the expansion has overreached a bit. Not that I'm having a hard time finding movies - I'm having a hard time returning movies. I've had several occasions where I've tried to return a movie, only to be informed that the kiosks was full and not accepting returns. This means I now have to drive to another location (usually the next Walgreen's down the road), and try again there, often to fail again. And of course, there is a time limit on the search - you can't wait and try again tomorrow without paying again.
The two points in favor of Redbox are convenience and price. This problem endangers both benefits. Perhaps they haven't hired enough staff to service their expanded deployment?