Left-mouse button click not recognised (intermittent)

I have an OptiPlex XE4 with wired Dell mouse.  After a period, the left mouse button fails to be recognised.  Rebooting clears the problem, but it then returns after a while.  I have followed all the advice on refreshing the drivers etc but I can't get rid of the problem.  This has only been happening for the past few weeks (possibly since a Dell update of some sort).

Any ideas?