Check that the chain is well lubed and free of stiff links. Shift to an aligned gear combination and back pedal while pushing the lower pulley forward to slacken the chain so it sags. Watch for any links that don't immediately straighten as they clear the pulley, or changes in the curve of the sag indication that the chain isn't as supple as possible.

Also be aware that even when all is good, you can sometimes get suck if shifting under load because the load pulls the chain down onto the smaller sprocket's teeth while it's still engaged on the outer. The side pressure can bind the chain so it doesn't disengage cleanly at the bottom. So if you need to shift under some load, try to ease the load for at least 1/4 of a crank revolution so the transition area has moved to 3 o'clock and won't be affected by the tension. (not always possible if you aren't thinking).

You can also reduce shift related chain suck by some careful filing of the affected teeth, but that should only be attempted if you know EXACTLY what you're doing.