Just brainstorming here, hoping I can stumble on something to help. When you put the dolly in the hitch coupler and pull down the Dolly's handle a bit, does it lift the tongue jack off the ground? I think it should, though I don't have that dolly.
This is farther out, but is it possible your trailer brakes are locked on? If, for example, the pin has been accidentally pulled out of the breakaway switch, the brakes will be fully on and will stay that way until the pin is reinstalled or the battery dies. On my unit, I can hear the electromagnets inside the brake drums humming when the brakes are on, so giving a listen up close to one of the wheels should tell you the answer there.
I hope by now you've already figured out the problem. If not, someone will be along with the answer. You're right that it should be easy to roll the trailer with the dolly on a nearly level surface once you've overcome inertia.
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator