I have this problem with Dolly and I tend to find a solution and then she goes off that too and I have to think again. So far the most long term successful thing to get stuff down her is plain chaff and a good dollop of dried mint that I buy from Progressive Earth on ebay. Like you I try to keep her diet as sugar free as possible. I do always damp it down as well - she absolutely won't touch anything that isnt slightly moistened.
The last couple of months the dried mint isn't a success either. I recently changed some of her other supplements for Equimins Biotin 15 and when it came it stank to high heaven of pear drops, I thought she would never eat it. Turned out that she loves it and now gobbles the lot down, whatever I put in it!.
But I dare say she will go off that at some point soon, and yet again I will be trying to get inventive with it all!