As an ex-technician and directv supervisor, here is what I would suggest:
First off, it could be a whole variety of issues. Most likely cable or something inline with the cable causing interference. Could also be the lnb (end of the dish mounted on the arm) or even the receiver's tuner.
1. if one receiver is working perfectly, but not another then test the receiver with problems using the connection of the one that does work. meaning - swap the receivers. If the problem receiver now works, it's definately wiring.
2. if both receivers are having the same problems, it's either a wire from the dish to multi switch or the lnb. however, it could also be a bad port on the multi switch. moving the cable from one port to another could help.
3. to test the wiring, run a new cable directly from the lnb to the receiver. I used to run it through the front door, or through a window. No breaks, connectors, or anything in the line. if the receiver now works, start tracing the old existing wiring from the dish down. everytime there is a connection in the cable (connector, ground block, multi switch, wall plate, ... anything) connect the new jumper wire at that location. moving to each connection will pin point where the problem is.
4. make sure that the wire you are using is rg-6 3ghz swept tested. directv signal will work on lesser wire, but it is not reliable. if you are having problems using rg-59 or 2.4 ghz wire, it very well could be your problem.
5. wall plates are famous for not being directv friendly. typically they are designed for cable systems and do not allow all frequencies flow through them degrading satellite signal or completely blocking it. The color on the inside of the wall plate (and anything else in the line ) should be dark blue.
6. last but not real likely, the receiver's tuner is bad. I have seen it several times be the tuner, but 95% of the time problems are from cabling.
hope this helps ...