I found that some cases support 7 keys, but normally they support only 5~6.
It's really annoying because some cases take 3 keys and cannot check an important key.
If you press Tab+S+E, the hardware doesn't check the Up key. This causes co-op to not work properly.
Please check this problem

0.2.6b

Key rollover depends on the internal wirring of your physical keyboard.
If you have problems with multiplayer on keyboard and don’t want to invest in an expensive keyboard or a game pad, adding extra (cheap) keyboards for other players can be a solution. Beware of trolls abusing their access to your player’s keys.

@RealShadowCaster Thank you for letting me know! After testing the keyboard, it seems to be the issue. :)
I’ll try using an extra pad as a solution. Thanks again!