track pad freezes

When I use my trackpad, I increasingly frequently find it freezes.

I turn it on first thing in the morning and leave it to sleep when I'm not using my desktop, and it seems to happen most often soon after I have come back to my computer after I have been away from it for a while.

I have to switch it off, and then on again, to get it working.

Can someone explain why it happens, please . . . and how to cure it !