Dan:
YES...the Hydro Check Valves can get gummed up, clogged with foreign debris, and just wear out.
Remove the valves, place them in a "penetrating solution" for a while, clean them out, blow them out real good, and check for free movement. If you are confident that you done all you can, re-install them and check out the operation.
If all went well, then things should be back to normal. If not, then Check Valve replacement is probably needed.
Here is some further information....
Check Valves