too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Since it's fairly new and was exposed to lots of mud and water, I'd assume it's just a question of being bound with silt or rust.
First double check that it is the shifter and not the cables or RD by drawing the wire away from the downtube like a bow string and seeing if the RD response smoothly. Then check the handlebar to frame housing loop by shifting up to low gear via the bowstring method and parking it there so to have slack in the cable. Slide the housing out of the shifter and frame stop and see if it slides freely along the bit of slack in the wire.
If you've isolated the problem to the shifter, try gently working the levers while keeping a bit of tension in the wire. You might also remove the cover and see what you can see. As a last resort disconnect the gear wire from the RD and remove the shifter from the bike, give it a long soak in hot soapy water agitating it from time to time. If that frees it, rinse well, with multiple rinses, shake dry, and flush first with alcohol to remove any water, then a spray lube to protects it's innards.
If a good wash and lube doesn't help, bring it to the local shop and see if they can save it.
BTW- if you have a friend with an ultrasonic cleaner, say a jeweler or dentist, these do wonders flushing dried mud and silt out of small mechanisms.