There are a number of reports of difficulties figuring out compatible RAM (regardless of SODIMM capacity) for the Dell XPS 15 9550 (as well as 9570). The pattern that seems to emerge is that these systems don't like RAM with low CAS Latency. The odds of being compatible seems to increase with at least a certain minimum CAS Latency (CL) timing for a given speed. The following is likely ok with the 9550:
2133 MHz - CL 15 or greater
2400 MHz - CL 17 or greater
2666 MHz - CL 19 or greater