I have 3 receptacles in my garage. One is a GFCI which protects all 3 outlets. I want to connect a new receptacle off the third receptacle (end of run) on other side of wall, and have it protected by GFCI. I connected black to black and white to white. But when I reset the GFCI, it won't hold/reset. When I disconnect the new receptacle, the GFCI holds. I made sure black was attached to brass side, and white is attached to silver side. What am I doing wrong?
Look for a ground fault
With the wiring disconnected, you should not get a reading on the multimeter. If you do, it means you have a short-circuit between that set of wires. Repeat this process on the new receptacle, touching one probe to the green ground terminal and the other to each the brass and silver terminals. Again, no combination should give a reading.
Your circuit is too long?
It may be possible that the circuit is just too long, and the resistance in the conductors themselves are causing the imbalance. Try connecting the new receptacle to the previous receptacle temporarily, using short pigtails (CAUTION: Working on energized circuits is dangerous.). Turn on the power, and try to set the GFCI. Make sure nobody is near the exposed receptacles, and don't touch, or get near them while the power is on.
If the GFCI holds, there is a fault in the wiring to the new receptacle or the circuit may simply be too long.
Warning: I haven't done any calculations, research, or testing on this theory, so it could be completely false. It's just a thought, but it's fairly easy for you to test.
According to the documentation for the Two-Pole QO®/QOB Circuit Breaker and Ground Fault Circuit Interrupter
Is the power on?
GFCI receptacles can only be reset when the power is on. So if you're trying to reset it with the power off, you won't have any luck.
@TheEvilGreebo has a great answer on this question here including a diagram showing how the wiring should be run. I would start by verifying that all your connections at the existing outlet and new outlet are correct.