Here's an idea. Disconnect the IGH shifter by unscrewing the adjuster mechanism that comes out of the axle on the right side. Then unscrew and remove the rod going into the hub. This effectively locks the bike into high gear. Now see if the wheel turns freely. If so the hub is either unadjusted or could have bad internals. The chain pusher should allow the wheel to spin freely on either sprocket, so if you can see something rubbing that could be your issue. I have a 3 speed hub on a 700c wheel that wouldn't work and the LBS worked on it for a while before realizing the hub is packed with 2 kinds of grease. I had read that these hubs originally used oil instead of grease so I put a few ccs down the axle and it works fine to this day. Every once in a while I replace the oil which has seeped out. Another possibility if the chain pusher does not rub and the shifter is disconnected is the cones inside the hub are too tight. There are a lot of YouTube tutorials on rebuilding an servicing SA hubs and one may address this issue for the wide range SA hub. Overtight cones will become destroyed and definitely bind the spin.