Are any of the other Boston UHFs weaker than normal?
WGBH-TV uses a two tube transmitter, there is always the possibility one of the tubes is currently out. I don't know, I don't work for them. Have you tried calling them to find out?
WGBH-TV is on the same tower as WBZ-TV, WCVB-TV, WSBK-TV, WGBX-TV, and WYDN-TV.
WBZ/WCVB/WSBK/WGBX all feed the top antenna, WGBH the one just below it, and WYDN one below that.