Here are a couple of things I can think of off the top of my head. The PVS-7B is pretty simple, but everything must be lined up correctly. Luckily, there are tabs and grooves in the various parts the lign up with each other. Since they are easy to take apart, many do so and don't assembly them back together properly.
First, make sure the rear lens are moving in and out when you adjust them. When I got my first PVS-7B, the rear lens adjustment tabs were broken. So it was out of focus unless I pulled the rear lens out. I ended up buying a replacement assembly. Just turn the rear lens adjustment and verify they are going in/out. If they are not, pull or push on the rear lens to see if you can get it to move and focus.
Second, make sure the front lens is screwed on all the way. The PVS-7B housing has plastic threads that can strip if one is not careful as the front lens has aluminum threads. It can also cause peices of plastic from the threads to come off and clog up the threads, so it feels like it is on when it really isn't. There is also an O-ring in the front lens assembly that can break and bind up the lens. Remove the batteries (so the unit doesn't accidentally go on) and unscrew the front lens. It has the standard thread pattern (lefty loosy, righty tighty). Make sure the retaining ring on the inside of the front lens is screwed down all that way. If it isn't on properly, it can mess with the focus. Then reassembly, use a can of air dust remover to blow everything out to minimize dust. Make sure all the parts line up. The colimator has a tab to center it. The intensifier tube has a groove which matches up with the body. The front lens just screws back on. It isn't hard, just take your time and be sure not to cross thread it. It is a tad tricky as you have to keep everything lined up as you screw on the lens. It isn't hard, but be sure to pay attention to what you are doing and take your time.
Lastly, do as stated above and loosen the focus lock ring on the front lens. Some are glued into place, so you may have to get the proper tool for it. You can usually see the glue if it is glued in place. But try it by hand to see if you can get it. I bought a tool specifically for that, it was $45 or so. I had one lens that turned effortlessly by hand, but I also have one that is absolutely frozen in place as it has been heavily glued in place. Of the 6 or so I have played with, most are in the middle and take a little effort to move them. If you got some surplus parts, there can be a ton of dirt, grease, etc in the threads under the ring where you can't see it.