It is easy to confuse what happens in modelling to real life. In many cases (maybe most, but my experience is not wide enough to make that statement) an N scale 6 wheel truck is driven by 4 wheels. So there is effectively less weight on each driving wheel, and traction is lost. This is somewhat offset by the fact that the loco with the six wheel trucks is often larger, and may weigh more to begin with.
Three axle trucks are less tolerant of sharp curves, but blind drivers come into play on steam loco, where the wheelbase is much longer I've never seen a three axle diesel truck with an unflanged wheel. The thing about a three axle diesel truck is that any unlevelness in the track on a curve (rail to rail, not a grade), can cause the truck to pivot, or rock up, kind of pull a wheelie. In a shorter truck, the wheel comes back down still with the flange is still inside the rail. With a longer 6 wheel truck it is easier for the wheel to come down outside the rail, and there you go.
As far as your loco, assuming it is very gently lubed, it might be doing the best it has ever done, depending on the cars you are pulling. Less than freewheeling wheels on cars can seriously diminish the pulling power of the loco. Also, if it is smal, and hence light, it isn't going to have a lot of pulling power.
Jeff