I can understand how frustrated you are. We've fallen pregnant naturally 4 times as well. Started IVF in Aug this year but then cancelled the cycle because my lining was all the sudden too thin. Never had lining issues before. My NK test came back fine as well.
Did you have miscarriages or chemical pregnancies if you don't mind me asking? Out of my 4 pregnancies, the first 2 ended at 7 weeks and then the other 2 were chemicals. Still haven't figured out why yet.