The WAAS signal is not transmitted by "conventional" GPS satellites, rather, it is transmitted from geostationary telecommunication satellites. Consequently the signal does not behave like the signal from the orbiting GPS satellite constellation. Also, there can ne a dozen or more GPS satellites in view with only 4 needed for a fix but there are only two WAAS satellites with one being needed for a  WAAS approach.

Bottom line is that is WAAS is intermittent then it is likely the antenna location or signal path to your GPS' RF section (line of sight, antenna, connectors or Coax cable).

Mounting the antenna inside the aircraft will frequently cause problems due to the fuselage shading the WAAS signal to the antenna.

If I hang a GPS antenna out the lab's eastern window I can get a good fix from the satellites to the northeast but no WAAS.

Regards
Brent Regan