CP, again I am so sorry sweetie, I am totally gutted for you, I was so hoping this would be the one. I had a look back over my cycles and this is what I was doing:

I started Prednisone in Dec 08 during a Colorado Protocol cycle, and pretty much stayed on it the entire time, even while not cycling. When I tested + for MTHFR in Feb 09, I started on low dose aspirin daily, mega folate (5mg) and high dose B6 and B12 (the folate, B6 and B12 are specific to MTHFR). It was also recommended Clexane from ET or from postive BT (I pushed my FS for Clexane from ET as I had never gotten to BFP before). Continued to take Prednisone while not cycling at 6.25mg per day (quarter tablet - I had 25mg tabs) in case of NK Cells (though tested neg on BT, I never did a biopsy but FS was happy for me to continue).
Mar 09 Cycle #8 FET natural, 2 blasts transferred, Clexane from ET. BFN
Apr 09 Cycle #9 IVF antagonist cycle converted to IUI. I had sneakily done Clexane every second day from O day. FS hadn't recommeded Clexane as it was not an ET.
May/Jun 09 Cycle #10 IVF antagonist cycle with assisted hatching, 2 blasts transferred (1 hatching blast rated 10/10, 1 early blast rated 7/10) Clexane from night of EPU.

For this I took low dose aspirin the entire time except for 5 days before EPU. I had Clexane from night of EPU. Prednisone - I had 25mg tablets, CD1 to trigger quarter tablet (6.25mg), from trigger for 5 days (full tablet 25mg), then from there half tablet (12.5mg). I did assisted hatching for the first time, I had previously never had a hatching blastie.

I don't know if any of this helps. Given your NK cells are high can you up your Prednisone dose? Or maybe stay on it on a low dose until you next cycle, your body might need more time to reduce the NK cells. I am not sure when you started Clexane, was it from EPU or ET? This might make a difference also, I always felt I had clotty AF, and wanted the Clexane for a good endo lining thinking by ET it was too late.

Oh I wish I had something else to suggest. I hope the other girls have some ideas, or your FS too. Wishing and praying for you xx