Marlins are a straight forward gun for the most part. Without holding this gun in my hand and getting to check it out for myself I can think of two things right off the top of my head. First is the clip. If the clip has been slamed into the gun to hard, dropped, or somehow the sides been squeezed together a little it can cause the round to be alittle stubburn coming out of the clip (by stubburn I mean just little more drag than normal and hard to detect by just unloading the clip). Try a new clip and if this isn't the problem oh well now you have an extra clip.
The other option is a new recoil spring. The reason I think this could be the problem is because you have no trouble getting the first shot off every time. If the recoil spring gets weak it will still shoot the first shot because your pulling the bolt all the way back and releasing it letting it slam the first round into battery. On the sec. shot the bolt is still coming all the way back but with the proper spring tension it's losing to much momentum to pick up the next round and get it fully chambered and into battery (remember it only take a few thousands to keed it from firing).
On the other hand if the spring is to heavy it may not be allowing to bolt to fully cycle. This can cause the lack of firing on the sec. shot because it's not allowing the bolt to slide back far enough to cock the hammer. But if the gun has worked well in the past, a too heavy recoil spring is almost certainly not the cause.
I'm going the say it's either the clip or recoil spring for the reason stated above. If you don't want to strip the gun to replace the recoil spring try the clip first. But if you want to try the cheapest sulotion first then change the recoil spring first ( recoil spring cost about $5, clip is about $35).
Hope this helps
DJ
