Make sure the heartline is set correctly for a flyer. Idk the value off-hand. I think it should be negative. Like -0.8 or something.
Look at pictures of a pretzel loop. I'm sure you're familiar with Manta.
To dive into the pretzel loop, you'll need to have a force zone with vertical G's that go from about 0 G's at the top to -4.0ish G's at the bottom.
Yes, that's an unrealistic value for negative G's; but for a flyer, these will actually end up being acceleration G's in the simulator. So it is safe. When you run it in the sim, check that the acceleration G's hit somewhere in the range of +3.5 to +4.5.
You might have to edit the settings in Newton to allow you to go as high as -5.0 vertical G's.
Hope this helps a little.