Without seeing your circuit most of what I am telling you will be conjecture on my part.
1. Check to make sure all power connections are attached (obvious of course, but I have been known to forget to do this)
2. Under Simulation settings make sure that set to zero is activated.
3. You never said how you were triggering your timer, but it requires a negative going signal on pin 2. I usually tie a 1K ohm resistor to positive and trigger with a switch to ground. This negative signal cannot stay negative longer than the time-out period of the timer otherwise it will just keep re-triggering and in a way oscillate in an undesirable way. You can trigger positive to, but I haven't had much success doing this in Multisim unless I invert and integrate the positive trigger pulse.
Anyway, that is all I can think of barring a messed up timer in your version. If you could somehow post a picture of your circuit I may be able to get a better picture of what is happenning and may come up with a better solution for you.
