I used to have a problem with cross combing in my 2 Warre hives. Check to make sure that you have the spacing right.
And now when making the top bars, I now cut some square stock on the diagonal and attach that to the bar so that the cross section looks like
Put a bead of wax along the point and your bees will draw out nice straight comb. Works every time.
I used 1 3/8 inch wide top bars (maybe should have used 1 1/4 for brood) and I also have the V shaped extension on the bottom of the bar. I didn't use a wax bead. I may have to try wax next time.
Thanks for the suggestion.