There are various reasons why your horse may be stiff on one side...
Uneven Saddle
Take a look at your saddle down the gullet. Do both panels seem even? If not find your local ASFA Accredited Saddle Fitter to assess your saddle and correctly fit it to your horse.
Uneven Horse
Many horses are naturally stiff on one side and they need more work on their stiff side to loosen and develop the muscles. This is particularly true for ex race horses who are trained to run in one direction.
Some horses are actually uneven in their backs resulting from uneven feet, injury, back pain or natural conformation.
If focusing on the stiff side has not worked after a few weeks, it is worth having your local ASFA Accredited Saddle Fitter to tailor the right saddle fit for your horse. It may also pay to have your local Chiropractic Vet take a look at your horse to see if there is not something else going on.
Uneven rider
Many riders don’t realise they lean to one side which can make it difficult for their horses to travel correctly. Your riding coach can assess your riding position and confirm any issues. You may need to see a professional for your own back to help improve your riding position.
Please feel free to Download ASFA Fact Sheet - Is your horse not going well on one rein?