With time and use, the set screw on a handle may loosen up. Using a hex wrench will allow you to re-tighten the set screw in the handle. Your faucet came with the correct hex wrench. If you can’t find the hex wrench that came with your faucet, most single handle kitchen faucets will use a 1/8th inch wrench, while most two handle kitchen faucets will use a 3/32nd inch wrench.

If this doesn't fix the situation, you may want to loosen the set screw and re-seat the handle on the stem to make sure it is lined up properly. Once corrected, you may re-tighten the handle to the stem unit and it should work normally.