Beat grid speed is different for different channels

same song on all decks? 

Was the song analysed in dynamic mode?

I've just tested and no problems here

Also, what version of the software are you using and pc specs (if you want more help you have to provide at least this basic stuff)

I've noticed this. You just have to use a mouse wheel (or appropriate gesture on a trackpad) to match the waveform zoom again. Switch to 4 deck mode and zoom in or out to a maxium/minimum zoom then they just re-adjust back to your preferred size.

 Edit: I've just realised that, on the SX2 at least, you can hold down SHIFT and use the rotary track selection knob to adjust the waveform zoom too.