Actually mine did the exact same thing that yours is. Between the ages of 2 and 3 she stayed the exact same size. About 3 1/2 feet. I'd had boas years ago but had forgotten just how slow they grow. It isn't a steady constant growth like balls. They only have one or 2 major growth spurts a year it seems.

With mine I alternated her feeding. She ate weekly for the first year, then bi weekly. I'd give her an extra rat some non feeding weeks when she seemed hungry. She let's me know.

Now she usually eats only bi weekly but once in awhile during one of those growth spurts, I'll still give her an extra (small rat) to fill her up. I also don't give her huge meals. For her size she could be taking smaller large rats but she gets rats that are more like a medium most of the time. Just enough to put a visible feed lump in her.

I'm not sure if temps may have anything to do with it but mine is in a Boaphile stack with balls and a Retic. Retic on bottom for the coolest temp, boa above her, balls above them for the highest temp. So my Boa is kept a few degrees warmer and she actually spends most of her time on the hot side as well. Plus being a completely enclosed Boaphile cage, the humidity is always spot on as well.

Maybe when the weather warms up a bit, yours will eat an extra rat one week out of the month if you offer one that's smaller.