You can change layouts in
Xmonad with mod+space or switch windows (in fullscreen) with mod+tab.
I encounter everyday the problem that these bindings stop working (I don't know what causes it). I'm able to change workspaces with mod+1 | 2 | ... | 9 and this problem occurs usually only in one workspace. I tried to solve it with moving all the windows from the workspace (where it stopped working) to another workspace but this didn't help. I solve it with logging out and logging in. Is there some way to solve it without logging out?