For a start, try spraying the rod that the handle is attached to with some lubricant. Push it in and out a few times and see if that doesn't help. I have done this to mine a few times and it helped.
A tip I found on the internet is to put 1-2 quarts of vegetable in your empty tanks (black or gray). The oil provides some internal lubrication to the valve paddle. I've used this, along with WD-40 on the handle as retired mentions, and it made a huge improvement.
Drill a small hole in the top of the valve near you would pull the handle. Then shoot some silicone spray in. It worked to make the valve slide with ease. Then he put a stainless steel screw in the hole.
Look on the Internet for a product called Kroil. It is a lubricant that is used in Industry to loosen rusted things. It has the ability to "crawl" into tight areas, and I have used it to loosen a number of items successfully.
Goodman
Spray the Kroil on the shaft going into the valve and give it a couple of hours and then try it. If still stuck, give it a second dose and wait a day.
Goodman