On some trapped antennas, there is a rare case that occurs that causes an intermittent high SWR only on a high frequency band, such as 15 meters, while the antenna has good SWR and is functioning normally on all other frequency bands, such as 10 and 20 meters.
In order for this to occur, the coil inside that trap must be in complete contact. So, the fact that the other bands are working despite the intermittent condition on 15 meters means that the 15 meter internal trap coil connections are just fine. If the 15 meter internal trap coil was intermittent, then 20 meters would also be intermittent while 10 meters would be fine.
In most modern trap construction, the trap outer tubing is a sleeve that is connected to the element only on the feedpoint side of the trap, to form a capacitor over the coil. The trap outer tubing is insulated from the trap inner tubing and coil at the other end of the trap (away from the feedpoint) as well as from the rest of the element.
When the intermittent trap outer tubing loses contact with the feedpoint side of the element, it causes a disconnected capacitor, the tuning goes way off and the SWR rises, only that band.
So, in this case, the intermittent occurring on 15 meters ONLY means the connection of the trap sleeve outer tubing to the element may be the problem. This can be caused by incomplete production processes, where a swaged piece of tubing has some residual plastic, paper, or machining oils that are making that part’s connection intermittent in the trap.
Disassembly, cleaning and reassembly of that trap, or in some cases simply loosening trap clamp turning it back and forth to make the metal bite and tightening the clamp, are valid ways to solve this intermittent trap problem.
Additionally if you are cleaning the antenna connections, use Penetrox-A to help with electrical continuity between elements.