You pressing something like S+D+num4 and realising that num4 doesn't work? This is NKRO issue.

You can read about it here if you want: https://en.wikipedia.org/wiki/Key_rollover

TLDR: Cheap keyboards are cheap because their design eliminated all technologies that weren't essential for basic operation. For example, the boards were simplified, and because of this, keyboards don't allow you to press more than two keys simultaneously in most combinations. These keyboards never was designed for gaming purposes and this isn't a fighting games specific issue - prob you can't lean and move diagonal at the same time in fps games too. You can go to any keyboard test and check which combination you can simultaneously press and which not.

You can't do anything with this. Give up. This isn't a software issue. The only you can do is to get a better keyboard. Every keyboard more expensive than 30$ have NKRO support. I believe you can find it in it's technical specifications or simply ask the vendor.

Possible temporary solution that may work if you're lucky enough is to try to find the place on your keyboard that allow you to press more than 2 buttons in every combinations. Back in the days i found that my old kb allowed me to use navigation keys for that. The bindings was weird but it works for me untill i got another kb. Maybe it'll help.