Normally this is something simple like just being a pint low. In any case, most bartenders can remedy the situation rather quickly. If a bartender (or even a qualified waiter or waitress) is not available there are quite a few "do-it-yourself" remedies. One of my favorites is a Cointreau Strawberry Basil Rickey...
2 ozCointreau
1 ozFresh Lime Juice
1Strawberryhulled and quartered
2-3Basil Leaves
3-4 ozClub Soda
Muddle the strawberry and basil in the bottom of a glass.
Add Cointreau and fresh lime juice with ice and top with club soda.