My guess -

Take out most of the lead.

Take out some of the up-elevon.

Try it. Repeat taking out elevon until speed builds up a bit.

I suspect that you are trying to fly it too slow. Every flying wing I have had anything to do with likes to be that bit faster than the corresponding conventional aircraft.

Check too that the fins are at 0* and not toed in or out.

If none of those work, take another look over the whole airframe. There is something there which is wrong. Bit like a mate of mine who put the wing on a rubber model backwards ( I mean TE first). Couldn't work out why it looked funny in the air - it still worked, kinda.

Dutch roll is a function of dihedral and fin area - so does not apply here...