What rank are you? I find a bunch in S+1-10.

My guess is you're running into problems with 1) a weapon's series power or 2) the rotation + time of day.

I can readily find matches at anywhere from 1500-2100, but I have heard complaints from a top level player that they got too high a power on undercover and stopped being able to reasonably matchmake.

Other more likely issue is picking rotations people aren't playing. I run into this with X rank as well - bluefin/mincemeat clams will not get as many people queuing as say mako/mahi zones.

Try to play at afternoon/evening times in your region and maybe try swapping weapons?

It feels like most players left except the stronger players, who have often higher ranks. I think you may have a better chance in the Japanese player base in case you are not there already, but that requires some VPN setup and causes additional lag.