Mix each flavour seperately first with low percentages. Add flavour little by little till you hit your 'sweet spot' on each.
Then blend and taste.
Depending on their chemical makeup, individual flavours can - and do - 'cancel' each other out. Could be the case here.
Could still happen when you blend them, is very tricky to balance combined flavours.
Some ' co-exist' with no problems, others tend to crowd each other out.
If it don't work, try with different suppliers ... been wading
through about a dozen or so vanilla flavours till I got the one that suits me.
Zend , patience, and the art of mixing