My kid trailer I have had for a few years has, in the past few weeks, been noticeably harder to pull. I checked it over and I found that not only are my wheel cones are a little loose, that I can spin the axle with very little resistance. When I say little meaning I can give the quick release lever a light flick and it will spin 4 or 5 times easily with the wheel in the vertical orientation (off the trailer). I'm assuming that this is because the bearings are dry, and since anything I have rebuilt myself, the grease (park tools polylub 1000) will give some drag to keep it from free spinning.
I have pulled it for hundreds (if not thousands) of miles to this point without really any problem. This summer it has been in the rain twice (~4 miles), but I noticed this new drag before.
Trailer (shaped object) is a Pacific Voyager II sold through Toys-r-us (best that I can tell). I don't know the service history (assuming none) and I'm the 3rd owner. When I got it I replaced the tires on it (20" Kenda road tires) and keep the pressure high (50lbs of 55lbs max). At around the same time I also upgraded the hitch system to a standard Burley one so I no longer have the spring in the system causing tugging.