Don't worry,
I had the same pb, I had a sim check with 4 other people, I was the only one who was flying constantly and the one with the more experience (not difficult because the other 3 were ab-initio and I was flight instructor).
I still don't know how but I failed.
The only reason I can Come up with is that they all did some sim on 737 before the sim check.
But now I am flying a jet and upgrading captain in 3 months.
So don't worry, keep going you will get there if you keep going.
Good luck.