I am not a boa person so I cannot talk boa specifics.

Where are the vents? Heating isn't just a matter of more power but of location and heat loss. The enclosure being wood will insulate well so some loss may come from vent placement. I usually like to place a vent low on the hot side and high on the cool side cool air will heat up on the hot side and rise and escape the cool side keeping it from getting too hot.

Thermometers are a picky item and I constant source of frustration for me personally. Typically a thermometer (ir gun or digital probe) have specs that read +/- 2ºF so the accuracy maybe bang on or 2 degrees high. Good ones are very costly (very very actually... ) the best on a budget is a LC one and use that to check the digital one you have. The stick one ones not stuck to anything just leave the backing and place it let it stabilize and check the digital against it. The average LC one I have found is +/- 0.5ºF.

It doesn't sound too much like the thermometer however check the vents and placement. The other thing to remember is that boas like pythons have heat pits and I'd guess that also like pythons that is connected to the optic nerve so it is processed by the brain as visual input so they see heat as we see light. So any heat source is a light source too, so red bulb or emitter is basically the same to the snake (assuming that the structures are similar... that is a best guess on my part) go with what works.

I found a fluorescent bulb to raise the ambient temp up two degrees maybe a bit more in a 24x30x18 enclosure it helps a bit with the day temps. The night time ambient drops a bit but I have UTHs for the hot spot. Have you given thought to a mat like a Kane waterproof mat inside the enclosure or a RHP (radiant heat panel) over the bulbs?

Adding a basking rock under the heat emitter (bulb or ceramic) might help a bit it will warm up and radiant heat during the night. Just guessing here too. I am tossing out ideas as the come maybe something useful will come from it or I'll give you an idea that will lead to a solution.
Alex