A "whiff" of graphite (Kadee brand shown here) squirted into the coupler box (can't see it here, it's what the coupler fits in), and on the coupler itself, will often fix a mal-functioning coupler.

Move the coupler around by hand to spread the graphite around.

The coupler on the RS-3 is sagging.  To fix it, put a shim under it.  Microtrains sells the shims, or you can use the top of a broken coupler as a shim.  Remove the coupler, place the shim underneath, and re-install. It's delicate work!