It is possible that your initial blade angle is set too coarse.

With a CS or Variable pitch prop, one of the purposes is to enable max TO RPM at takeoff. One of the reasons that the RPM builds up after a few seconds is that after you achieve some forward motion the angle of attack to the blades changes enabling full RPM.

There is another possibility, again relating to the prop and that is that because of the pitch, the prop is actually stalling producing little lift (thrust) until forward motion changes the angle of attack and it unstalls. This is accompanied by a rather strange sound coming from the prop until it unstalls.

I  had that problem with my dragonfly. (Midwest AE110R/ CS prop (airmaster)  warp drive blades) Initially when I tried it on an early x-plane I thought that it was the program, however in real life it happened exactly as simulated. I solved that problem by slowly advancing the throttle.