I don't know if you're doing this too possibly, but here's what I've been seeing with beam BWO's lately. This is with a group of girls who are just learning them on the low beam. They had practiced on a line on the floor for about a month prior.
My gymnasts on floor do them great, the part where their legs are split and going over they are holding their cores tight with zero back arch. They lever out nice and straight and finish. On beam, (2 gymnasts) arch their backs during the phase where their legs are split and weight is on their hands. Sometimes so much that it looks like their rear ends are going to touch their shoulders! They explain that this slows them down and gives them time to place their feet on the beam. It's got to be murder on a back though!
In response to this I had them do BWO's on low beams with panel mats on the side for them put their hands on. All they did was get their feet to the beam and finish properly with the core being held tight in their vertical. They did very well with that, next week I'll switch it. Panel mats for their feet, correct beam hand placement and no severe back arch.