GoldenRivet summed it up nicely. Submarines don't rely on forward or backward motion to propel themselves upward or downward, they rely on the displacement of air and water.
A theoretical situation where a submarine might "stall" is if there is heavy flooding and the sub is making an ascent at a steep angle (as would be likely in an emergency). As flooding increases, it displaces the air inside, and the submarine relies more and more on its own propulsion to carry it to the surface. If the flooding reaches a point where propulsion can no longer drive the submarine "forward"...well...start writing letters to wives.
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