New GP antenna, insulate from Mast?

[SoCalRVer]

I recently moved my radio shack from my motorhome into my house. I mounted a 4 foot piece of EMT pipe to a vent pipe on my roof and attached the antenna. The antenna is an Arrow GP 144/440. It looks like a J-pole with radials pointed down about 45 degrees. I used tie wraps to secure the RG-8X coax to the mast. My radio is a Yaesu FT-1900 mobile.
I am getting out fine to the local repeater which has perfect line of sight. Using the radio for simplex is interesting. I seem to be getting out fine except that when I check in on a Net, I am able to get a good signal from all except the Net Controller. He is about 5 miles southwest. Another station responding is another 2 miles away and also southwest. I can read him full quieting, great signal.
My question is, mounting the antenna to a metal mast that is probably grounded thru my sewer line causing me lose of signal? If so, how can I insulated it? Also, can turning the antenna with the J-pole section a certain direction help?