I had this issue last year and decided it was one or more of 3 things:
1) Super cold in my garage at the time, like sub freezing, thought maybe belts were stiffening and slipping (don't know where you live, but I'm guessing it's not that cold this time of year).
2) Springs too strong. I think the stock springs are over speced, but I'd done deeper cuts without issue until this point (Dewalt router, longish bit, router seated all of the way down in holder).
3) Dust boot. At the time I was using a router mounted dust boot and cutting small, but deep pockets. I think the bristles were fouling on the material outside the pockets.
Solution: I readjusted belts, but none were loose. I adjusted v-wheels, but none were really out of adjustment. I slowed my plunge rate of my cut, no joy there either. I never messed with the springs, but I had cut deeper without issue before. And finally, I took the brush off my dust boot and all was well.
I think looking back there's projects that a dust boot that moves with Z is a good thing, like shallow cuts, cuts where you need to dodge your clamping solution, etc.. But I also think there's projects where a fixed height dust boot is more appropriate. So I bought one of each. There's probably other projects where a dust boot just gets in the way, and I'll leave it off and just clear chips with a vacuum in hand.
Just my $0.02, YMMV.