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.
i7-2600K, H70, P8P67-M Pro, 16 GiB, HD7950, SSD, 3 HD, Blu-ray, X-Fi Ti Pro, Antec P182, S75CF, 3007WFP+2001FP, RK-9000BR, MX518
i5-3570K, GeminII-S524, P8Z77-M Pro, 16 GiB, HD7770, SSD, 2 HD, Blu-ray, InifiniTV4, NSK2480, 55" TV; Asus UX32VD