Sure it could be a ground issue, or bad bulbs, or bad sockets, or a mouse chewed it all to heck. If you think you have a ground issue what I've done is grab your jumper cables (or any long wire) and use those to supply a ground path from vehicle to the trailer. I had a tilt two place that was giving me fits and the cables worked great because I could move the ground to different areas to figure out where the ground was open. I usually will add on a wire permanantly attached to trailer with a clip on the other end to hook up somewhere on the truck. This way you can un-hook it easy. This not only helps lighting issues but seems to make electric brakes work better. Good Luck.
