Yes.Now would you believe it!
I've been trying to for some time to see if there is any consistency in whether alerts are triggered when they should be, it affect all alerts (I've not been testing media related alerts).
In the case of watched threads the reason for watching seems to be immaterial
I've ruled out being simultaneously logged in on two systems and the popularity of threads which might send more alerts to systems than the server can handle.
Resetting the options seems to help for a while.
Strangely 'Can I...Should I' @Sam Luvit seems to be immune from problems.