Your problem is the switch. It has to close, then open before the end of the pulse. I don't use Multisim, but all versions of Spice have a voltage-controlled switch which you can use. You may already be using one, but not controlling it correctly.
Your other alternative is to place a differentiator between the switch and the monostable. For an example, see the third schematic from the bottom of the page (555 timer Mono stable (one shot) circuit) at **broken link removed**.