Is my preamp failing?

I have two UHF and one VHF antennas connected to a preamp. This has worked fine for 5+ years.
On one UHF antenna (it is a 4 bay antenna) I receive 4 tv channels but now on some evenings (after 10 pm) two of the channels will lose signal.
Using the signal strength (in diagnostics) on my Samsung TV I could see the channels that had problems showed as 11 to 14 db and the ones that locked where at 25 db and up. If I unplugged the preamp the good channels would lower about 10 db but the bad ones stayed the same. Not sure if this test proves anything or not.

This problem comes and goes (currently it is working ok - one week it happen two nights then a few nights the next week then ok this week)
So can a preamp work for some channels but not others? Looking at the list below the 33.1 & 55.1 are the ones having problems.
15.1 - RF channel :31 - 34 miles at 221 degrees Antenna - zone blue
21.1 - RF channel :24 - 34 miles at 222 degrees Antenna - zone blue
33.1 - RF channel :18 - 34 miles at 222 degrees Antenna - zone blue
55.1 - RF channel :36 - 34 miles at 224 degrees Antenna - zone Violet
39.1 - RF channel 40 - 34 miles at 223 degrees Antenna - zone Violet

Looking for ideas on what to check next to see what is the problem.

Thanks
Tom