Too vague a question you are going to need to be specific giving examples and full system specs.
It's hard to guess...
In my experience with crossfire (2 x R9 290) I suggest you to do the following:
- Use latest Crimson 16.8.3
- Make sure that both cards run at full speed when gaming (e.g. use MSI Afterburner to show gpu usage and gpu clocks in game)
- Use windows power plan "High Performance"
- Make sure there is actually a crossfire profile for your game
- Make sure that those problems don't occur in single gpu mode, so diable crossfire globally and have a look at your games