Tab brake lights.

Did it work before? Might try a little dielectric grease on your seven pin connector. Check both the tow vehicle end, and the trailer pig tail for corrosion first, put a small amount of grease on both ends after cleaning any corrosion off. Lots of times it's just a poor connection on one or more pins in the connector(s). Might want to check for corrosion in the light socket for corrosion, as well as the wires leading to the socket to ensure they all have a good connection. Might be a silly question, but did you replace the bulb with the same # bulb you took out? Check the # for the bulb on the other side, and make sure they're the same #.  Just a thought, hope this helps you out. 

I had that problem and it was a loose wire in the 7-pin receptacle. (As ScottG mentioned)

I've had the same problem too, and with all-LED bulbs, it can be worse.  They take such little current to drive, any amount of corrosion blocks the power.

I've had more luck using contact cleaner (brake cleaner) to clean off the contacts, and then using a file to remove any built-up corrosion, following another rinse with contact cleaner.  I did this on both the T.V. and the trailer harness.  Of course your mileage will vary (YMMV).