LT3798 Power Output Too Low


I've designed and built a circuit based on the LT3798. It's very similar to one from the datasheet. I need to input 90-240VAC and output about 4A @ 24VDC (about 100W). I've got it to work fairly stably but it only puts out up to 40W before the output voltage drops. I'm using an almost identical circuit to the one from the datasheet but it's been adjusted for an output of 4A instead of 2. I'm also using a custom transformer (4:1:1). There are two main problems:

1. The output voltage drops once the load gets to about 30W (~1.2A). It's stable up to that point.

2. The snubber circuit gets very hot regardless of whether I use the one from the datasheet or from the reference design.

Here is an image of the schematic. Ignore the dots on the transformer, I just used a generic symbol from my library. I'm doing my rectification on another board, and that part works fine but right now I'm running this from a 120VDC power supply. I've also added about 2mF of output capacitance. In the interests of not making this post too long, I'm going to stop here so if there's something you'd like to know, just ask.