Have you tried just pulling the latch up all the way, beyond the point needed for opening the gun, and holding it there while you turn the cylinder backwards?
The 999 was the top of the line for H&R, and underwent a lot of small changes over the years, but still suffered from the old type lockwork and H&R's inability to find capital for a "clean paper" redesign. The trigger pulls were never as good as that on an S&W or Colt, the chambers were not always in alignment, the barrels were not always as good as they should have been, the hammer fall had a lot of "bounce", the grip was never as good as other DA revolvers, etc.
A big problem was that the 9 shots resulted in a fine ratchet that wore out easily.
Jim