In your case, probably, timer resides on the form. When form lose the focus, Timer event (oTimer.Timer) will not be triggered.
I solved this problem creating App property:

*-- Create the Alert's timer
goApp.oAlertTimer = NEWOBJECT(ccOBJ_ALERTTIMER_CLS_NAME, ADDBS(JUSTPATH(.CLASSLIBRARY)) + ccAPP_CLASSLIB_ALERTMANAGER)