Hello , my name is Jack Perhaps I can help you with this problem ;
Without being there myself with tools it would be difficult to say what is happening but what i would suggest is perhaps to remove and re-install the stand to see if that improves the situation and perhaps apply corrective measures in the form of shims ??
Second and more viable option would be to remove the existing stand and mount the tv on one of those universal stand like you can see and buy at the link below , just click on it;
Or this one;
If that answers your question please do not forget to rate or you can use the reply button before or after rating if you have other questions , thank you