What to do if expediting your ascent or descent is impossible/unsafe?
You report unable, and ATC will come up with a different plan.

To provide some context: If I ask you to expedite a vertical manoeuvre, it is probably because you are on crossing tracks with another aircraft, and I want you to pass them either above or below. This could be to meet certain level restrictions, or simply because I want to provide you with continuous climb/descend as a service. At your current rate, you will probably pass them, bot not quite with the 1000 ft vertical separation I need. Because you are so close to fitting into my plan, asking you to expedite is the easiest - it requires the least effort from me, and I won't have to vector you around, giving you a longer than needed flight path.

If you respond with unable, I have other options to choose from, depending on the situation. Typically I would either instruct you to level off or turn. If you are descending through FL160 and I wanted you to go below someone at FL130, I would simply instruct you to level off at FL140, and then descend you further when you are clear, which would only take a few minutes. I could use the phrase "Stop descend FL140, expect further descend in 3 minutes", in which case you could adjust your vertical rate so that you reach FL140 about 3 minutes in the future, resulting in a smooth descend. Alternatively, a turn of 20-30 degrees is often enough to establish lateral separation, which then allows you to descend unrestricted. 3 or 4 minutes later, when you have passed the other traffic, you will be turned back on your track.

The key thing here is communication. As an air traffic controller, when I instruct you to do something, don't think that I don't have backup plans. We are taught to always evaluate situations thoroughly, and come up with 2-3 different solutions to every problem or situation we face. I will start by presenting the easiest solution to you, but if that doesn't work for you, please don't be afraid of informing me - I will then simply choose another solution, and adjust my plan accordingly. And remember, air traffic control is a service. We are here to make your life easier - not the other way around. You can help us doing so by communicating clearly and in due time.