Hi Burr, Danny's analysis is right on.

The pieces are just slightly off center in a couple of different ways, that tends to create difficulty with producing an intersection right through there.

Like Danny was showing, the 2 pieces don't quite align at their juncture, and also each piece is itself not exactly lined up on the base line:

Viewing from the Front, towards the left side of the model:




Viewing from the front, towards the right side of the model:




Notice how the left side is kind of above the base line and the right-side is below it? The piece is basically not all aligned on that base line.


One way it might have got like this is this one curve piece seems to be sticking out downwards a bit:




Once you get one piece kind of misaligned, then if you mirror it and snap the mirror points on to the misaligned piece, your mirrored result will also tend to get a slant to it.


The other thing is that it looks like you want to have half pieces at the end? If you have modeled half of it, you should be able to solidify that half by selecting it and using Construct / Planar to seal off the planar opening (if it is all aligned and planar that is), and then mirror that solid half-piece to get your other solid half-piece, rather than sort of gluing it into a solid then cutting it again...

I'll see if I can do something to line up the pieces.

- Michael