Good advice already here I think but I will throw in my $.02:
Some of those guns are, I understand, pretty ammunition sensitive so the ammunition issue mentioned by Subgun and Captain may very well be the problem. Take a look at this thread
here which discusses needing full power loads to make them work reliably and that some off the shelf pistol ammunition won't generally function correctly. Did it used to work reliably? If so, was it the same load as is giving a problem now or something different?
Along the same line, were many lead bullets fired in it? It is my understanding that lead bullets could foul the gas system and that maybe even the Ruger manual in later years of production suggested jacketed only? See
here. It may be that the gas system is fouled and just needs to be cleaned to get a complete stroke and give the bolt enough force to feed correctly.
The recoil spring mentioned by Mr. Bowser is another possibility, as springs do wear out on older guns and it sounds like it doesn't have the "umph" to get the job done. However, if the gas system is clogged or it is an ammunition issue, a new spring may just make the problem worse.
As Captain, dhollis, and nonnieselman mentioned, Billy Tierce, 4050 Jack Rd., Utica, MS (601) 885-9297, is a good 'smith and I don't think he would treat you wrong.
Finally, although parts are hard to come by for that model, they are available. Numrich is a very good source and it looks like they have several pages of parts for different carbine models:
http://www.gunpartscorp.com/catalog/Models.aspx?catid=382
Good luck.
