Are you having issues on the ground or under saddle getting that perfect side pass or even with a half pass?
Take a few steps back and ask yourself this:
Does my horse understand the cues?
Am I cuing my horse properly?
Is my horse in good enough shape to do this?
Am I riding my horse in a balanced manner to allow him the freedom to move the way he/she needs to?
Does my horse understand how to move his/her shoulders independently from the rest of their body?
Does my horse understand how to move his/her rear end independently from the rest of their body?
Does my horse collect well and truly give to the pressure?
If your horse is not well muscled and needs to get in shape consider groundwork including advanced lunging techniques where you ask them to change directions often to build their muscles. As well, you can teach your horse to side pass on the ground while lunging! Start by asking for a half pass while you are lunging your horse by stepping towards their ribcage, while you are angled towards the shoulder with increased energy - watch to not be kicked or to step too close! If you horse takes any step away from you stop the pressure and reward your horse, ask again. Repeat this until you can have your horse half passing a few steps away from you when you walk towards them while lunging. Eventually you can turn this into a true side pass on the ground. If you can get them well muscled for the maneuver on the ground before you even involve yourself you will be ahead of the curve!
Under saddle the majority of the time if your horse understands the cues, is well muscled and ready to work, unfortunately the mistake is coming from your end. Do NOT be afraid to ask for help! Have someone photograph or video you to make sure you have your core, hands, and legs placed properly. The most common reason why lateral movement fails is because your outside or supporting rein is failing. Try a few adjustments and see what happens, but remember to get an outside perspective from a friend, fellow rider, trainer or instructor!
If you have the opportunity to ride a well trained horse and feel the maneuver then do it! This will allow you to learn what you feel when your own horse does it so you will be able to release him/her quicker and reward them for doing what you have asked! This is a team effort - invest in yourself as much as you invest in your horse and it will pay off!
Suggestions: If you want your horse to become even better balanced and well muscled, plus softer under saddle then get them softer on the ground! When you are lunging they should maintain an arc to the inside with slack or a "smile" in the line. This is the equivalent of them travelling softly under saddle in a circle when you can see the corner of their eye and often part of one nostril. Remember you will not accomplish all of this in one day and do not expect your horse to either - practice, practice, practice!