It’s very, very rare, but it happened several times that during battle for some reason I couldn’t weapon swap. Even pushing the weapon swap button next to the skill bar didn’t work. To fix this I have to go out of combat, unequip and re-equip a weapon.
I don’t know what causes it, so I’m sorry I can’t help reproduce the bug; I can only say that I keybind the weapon swap to the wheel button of the mouse.