Essentially it comes down to how these kind of keyboards are engineered.
Take a look at this diagram:
Basically your keyboard is made of a bunch of criss-crossing wires. When you press a single key, a unique pair of wires receive a signal from it. The key press is then determined from which wires are activated.
This works fine for 2 keys as well. Consider in the diagram above if the key E is also pressed. Both keys can be determined from this.
But problems occur when you bring a third key into the mix. Now consider what happens if you also hit W. No new wires receive signals, since both the wires connected to W are receiving signals from the S and E keys independently. Thus, the keyboard can't recognise a new keypress, and ignores the W key.
It's a little bit more complex than this simple diagram and explanation, but you can see why certain key combinations don't work with this kind of keyboard. Unfortunately, since this is a hardware issue, the only fixes are "change which keys you need to press" or "get a better keyboard".