If it is just one side that is usually a sign that one of your bulbs has burned out. Check your turn signal bulbs. It seems that the rear bulbs are more prone to failure than the front ones.
If it is both sides and the bulbs are OK, have a good look at the turn signal switch. This can happen when the contacts get worn or dirty. You may need to replace the switch. Sometimes you can resolve it from pushing the button rapidly on the 4 way flashers as well as repeatedly turning the turn signals off and on to clean the contacts. Make sure that the button for the 4 ways does not stick part way between off and on.
