There are times where the tonearm lifter may drop too fast or not drop at all. Usually a simple adjustment is all that's needed.
If the tonearm lifter isn't dropping
You can free up the t-bar by loosening the flathead screw located on the top of the mechanism, and then moving the plastic part up and down a few times.
You then gently tighten the flathead screw to return the t-bar to its original position.
If this doesn't solve it, it's likely that the lever isn't catching correctly.
If the lever isn't catching correctly, the unit will require a physical repair.
If the tonearm lifter is dropping too fast
The thing that gives the tonearm lifter a nice gentle motion is the damping fluid along the vertical shaft of the t-bar.
It's likely that the fluid has dissipated a bit too much, so you will need to add some more. Grease It is the perfect solution for this. You can also use other aftermarket high viscosity silicon fluids.
To get more fluid into the shaft, you'll need to unscrew the flathead screw holding the t-bar onto the mechanism, remove the t-bar (be careful not to lose the spring) and apply the fluid inside and outside the t-bar shaft.
When putting back together, the number of turns applied to the screw changes the t-bar resting height, so this will need to be set to work correctly with the tonearm position. It's not something that will need to be done regularly – if it's done nicely this time it'll be good for years to come.