Practice, first and foremost.

Chances are the transition between the two shaving methods has started to round out the edge a little.

Strop diligently. Tighten up the strop. Try 200 passes with a touch of pressure followed by 200 passes with no pressure. Focus on using the same pressure on both sides, equally, and use a light touch overall. Your not trying to force the edge into compliance, your trying to gently erode the problem away.

When you shave, search for and diligently use the correct angle. Improper shaving angle combined with pressure will destroy a well honed edge.