Excell is so moresome I dont think you'd ever get them onto A&P with the 50/50 method. Mine resisted it that way too.
If I remember correctly I stopped the Excell completely and gave them plenty of hay and small amounts of A&P building it up as it was accepted. All sorted within a few days although I left my old girl (at the time) on the Excell.
Economically it makes sense to feed it if you have any number of rabbits. I always recommend Excell for 1 or 2 rabbits but mine thrive on A&P.
To jazz it up a bit I used to mix it 3:1 with A&P Herb and Fibre but my supplier is struggling to get that atm and they still eat the Natural with no problem.
Worth persevering with - good luck
xx