It may be specific to your particular keyboard. One of the things to look for in a gaming keyboard is "n-key rollover." The keyboard may be limited in how many simultaneous keypresses it can track. A keyboard with true n-key rollover (like my CVT Avant Prime) can keep track of every individual key, up to all 100+. Usually, a 4-key or higher rollover works well enough, but a 3-key rollover is not enough for some games.
i7-4770K, H70, Gryphon Z87, 16 GiB, R9-290, SSD, 2 HD, Blu-ray, SB ZX, TJ08-E, SS-660XP², 3007WFP+2001FP, RK-9000BR, MX518