Older H&R revolvers are known to come down with weak mainsprings. A recently purchased 999 of mine was no exception. ( It fired consistently in SA only. Da produced about 50% FTF.) I tried hunting up aftermarket performance springs for H&R's but had no luck. ( I'm sure you could get one to fit but picking one would be trial and error. ) I eventually ordered a replacement from egunparts. That one came in too short and wouldn't produce even reliable SA firing. Trigger pull also didn't lighten much .I eventually precompressed the original with a few washers and it works fine now. ( Even if the pull isn't better.)
I bore you with this story because: a) I tried and failed to come up with a better spring b) you should know that these guns require some decent power in that spring to work correctly. About the only thing that may help ( IMHO) is some polishing of internal parts ( to reduce friction) and Then some custom spring fitting. While I do like my 999 , I've realized it will never match the trigger train of my S&W's. ( If only S&W made a breakopen "lemon squeezer" rimfire :-(
I'd say that 44 man is right on for the value of a 929. The 999's ( as H&Rs top of the line) may draw a bigger price if condition warrants.
P.S. If you do find something that helps, I'd be very interesting to know.