First thing to do when trouble shooting a light that is not working right is to jiggle the connector. It may be that the contact in the connector are dirty or the connector is loose
Next, check the bulb to be sure one of its filiment(s) are not burned out or broken.
If you can't see if the filiment is ok or not simply remove the bulb that is not working correctly along with the same bulb on the opposite side of the viehicle and exchange them and see if the problem goes away. If it does then you just need a new bulb. If it doesn't then there are several things that could be the problem.