Above post is pretty much correct, though your keyboard doesn't necessarily suck. It IS your keyboard's fault though, this is known as Ghosting. As far as I know, the only way to get a keyboard that will not do this for sure is to get a "mechanical" keyboard, but those cost a crapton. There are some keyboards out there that aren't mechanical that claim to be "anti-ghosting", and they generally are pretty good at avoiding these kinds of problems, but some still suffer from having very specific key patterns that just don't work for it. My CURRENT keyboard for example is a pretty darn good keyboard, I really like it, and it doesn't ghost even if I push like 8 keys at once of various locations... HOWEVER, it just doesn't like shift, z, and arrow keys together.. like I can push any 8 other keys, but shift, z, and ANY arrow key.. can't do it... fuck.
My last keyboard was a pile of dung, it ghosted nearly all the time with countless key sequences.. however it could handle multiple arrow keys, and shift, and z, and x all at once.
Problem is if you go to a computer specialty store and ask staff how a keyboard ghosts, they will most likely not have any experience outside of playing a WSAD game, seldom do people in NA (assuming you live there) play with z,x,shift, and arrow keys. So your options are limited to keyboards you can actually TRY before you buy...or a mechanical keyboard. Over here they are all like $140+... qq.