Unless I have misunderstood your question, the stick is NOT supposed to centre like what you get from a sprung Warthog for example. This replicates real life stick behaviour where the stick is physically joined to the flight control surfaces. i.e. the stick position aligns with where the ailerons and elevator are physically positioned. YOU centre the stick.
Also, whatever position you hold the stick for a particular attitude of flight, you can "trim out" the forces on the control surfaces and stick will stay still, you can take your hand off the stick to prove you are trimmed out correctly. Welcome to real flying! Hope that's helpful.