Ok .. so I managed to get some video of this .. 7 min fight vs RoL Winter Soldier ... a good 17 sp2s ..

4 of which fail (1 was my issue .. other 3 are bit harder to explain)

https://youtu.be/vv8qRuwF1kY00:51 sp2 intercept . no combo. (was too high on power to risk combo)
01:06 5 hit + sp2 - works fine
01:37 3 hit + sp2 - ok
01:50 5 hit + sp2 - ok
02:12 4 hit + sp2 - ok
02:30 5 hit + sp2 - ok
02:50 4 hit + sp2 - fails - my bad, was bit slow on the commands

03:08 5 hit + sp2 - fails
03:27 5 hit + sp2 - fails
03:40 5 hit + sp2 - ok
03:52 5 hit + sp2 - ok
04:10 5 hit + sp2 - ok (against wall, never any issues against a wall)
04:40 5 hit + sp2 - ok
05:26 5 hit + sp2 - fails
05:49 5 hit + sp2 - ok
06:10 5 hit + sp2 - ok
06:33 5 hit + sp2 - ok
I've been noticing this issue lately as I use Mephisto a lot ... 5 hit combo (not against a wall) .. into sp2 .. sometimes the sp2 kicks them too far back, and they're able to block or avoid the rest of it.
If there's something I'm doing wrong, would love to hear it .. however, looking it over, the only thing I may be guilty of is hitting sp2 too early .. that shouldn't be resulting in it missing like that regardless ..