Sometimes heroes in your deck (usually not active heroes) are not fully healed at the start of the build round. meaning you have to sit there for 10s holding down 4. not that enjoyable.
This is actually intended. Trendy wanted to put a stop to people just changing heroes over & over just to stay alive. Although why they decided that is a good idea is unclear. I suspect it was part of an effort to make the game "harder" & get us to start thinking harder on how to play & where to be and when to be there.
I hope they change their mind about this. Or at least heal our inactive and indeck heroes at the start of each wave.