Timer callbacks, the same as user actions on UIR controls, are processed only if your program does permits this: to do so, you can add ProcessSystemEvents () instruction regularly in your code, so to let the system process all pending events of your application. Look at the on-line help for that function for some more details on this.
Another way of firing events at regular interval can be to use an asyncronous timer, using the AsyncTimer instrument shipped with CVI: this timers are executed in another thread and rely on multimedia timers which have higher priority in the system, so their action is never interrupted.