Walk Behind Lawn Mowers

 I have model HU775H and am havinf issues with the rear wheels locking up when pulling mower backwards.  It seems the only time they do engage fine is when it is a hard flat surface like a driveway otherwise both wheels lock up.  My first purchase was the HU775L and I had the same problem and had taken the mower back to get a different model but am having the same issues.  It's not very easy to drag this mower up hill backwards when both rear wheels are locking up.  Is this a problem with Husqvarna Mowers?