When people learn Colemak, sometimes they unconsciously try to "ground" their fingers on the home position, and that makes typing very strainful indeed when typing on the middle columns.

When typing on the middle column, make sure that your fingers float freely, and the middle and ring fingers should move along with your index finger, e.g. when pressing the 'H' key, the middle finger should be hovering approximately between the 'J' and 'K' QWERTY position.