I've been using one of these Ergotron standing desks for a few years now: Ergotron WorkFit-S 33-341-200 . The maximum height it'd raise to has reduced over time; it took me a while to realize it as this was quite a slow process.
It turns out the fix is really simple. I found this entry in the Ergotron forums: WorkFit-S Dual Maximum height.
>The main point is in the second comment (a reply from an Ergotron tech):
One thing that you can try is "resetting" the WorkFit-S. To do this, lower it with one hand on the handle & one hand on the workstation surface - when it gets to the low point, push on the workstation surface a little bit - you should hear the sound of the desk "reset". Afterwards, you should be able to have the full range of motion that you had before.
This worked well for me - at least after I realized the 'workstation surface' was really anywhere on the movable arm (my Ergotron doesn't have a workstation surface). I had to press quite hard, the arm moved further down than it's base resting position and then had the full range of motion back. This meant that standing was comfortable again (before, my arms were bent as the keyboard tray was too low).