We know that the spring (likely a pneumatic cylinder) hasn't gotten stronger, so the hold-down mechanism...actually a valve...must be faulty.
Where the adjustment lever enters the square tube you may or may not be able to see inside the tube. At the very top of the pneumatic cylinder there should be a valve that is depressed when you move the adjustment lever. This is just like an office chair. Pressing down on this valve opens it and releases the "lock" function; the table rises.
If the adjustment lever is itself adjustable, you may be able to make it put less pressure on this valve. If not, it's likely that the adjustment valve has failed. If this is the case, you need to replace the pneumatic cylinder.
Unbolt the square tube from the tabletop and you should be able to see inside and look at this problem more clearly.
Anson & Donna
Be kind whenever possible. It is always possible. ~The Dalai Lama