a friend of mine has a Ruger P94. last month he bought a 30rd. mag at a trade show. it will only hold 12 rounds of .40. my best guess at the likely culprits are: stiff spring just needs to be worked down, FBO in the mag needs to be taken apart and cleaned, only holds 30rds of 9mm and wasn't properly labeled at the show. what do you guys think?

My best guess is that unless it is a Ruger mag. It will not work with the full capacity claim. I have bought many after market mags. that claim high capacity and will either not work with the amount claimed or won't hold the amount that it was supposed to be.  
Summary; BEWARE of aftermarket mags. that claim to be High caps.
I bought some of the South African mags for my Browning High power that were supposed to hold 17 rounds.  They were hard as hell to put 17 in and when I did, they did not function unless I put only 13 or 14 rounds in it.

My suggestion would be : stick with manufacturer mags. for positive dependability.

Most aftermarket hi-caps are junk, for lack of finding a nice way to say it.
Don't feel too bad, I bought a few in the past only to find out of spec bodies, bad springs and soft feed lips.
Not to mention bad finishes and they rust easily.
Even if you do manage to get them to work, save them for range use as I wouldn't count on them for serious social encounters.
Maybe next year, a quality manufacturer will make some that function...they did exist, I have several 20 rounders for the Ruger that are very well made and have been in use for years with no problems.

he bought it for range use only, since the clip hangs out of the bottom of the gun about 8". Anothergene, who makes the Ruger clips you have or where did you get them? if you don't mind me asking.

I got my nickeled 20 rounders from CDNN years ago, when they were $29 each.
The next catalog raised the price to $49, I think, and then they were gone shortly thereafter.
The floorplates and followers even interchanged with the original Ruger P-85 mags.
Not being sure who actually made them, all I can say is the bodies are very well done, heat treatment, finish and dimensionally.
They only hang down an inch or so and the one I use all the time has been flawless.
The others reside in my safe, along with "factory" S&W 20 round 9MM mags, amongst others and remain NIW.