Indeed, it' the combination of key's you're using that the keyboard isn't liking. There are two things you could try.

1) If you're uing ZSNES, somewhere under one of the menus is an option to allow you to assign complicated key combinations to one button, just use that to assign one button the equivalent of pressing each of the 3 code keys, then releasing them again afterwards.

2) Change which keys you use for which button. I'd suggest change at least one or two to use SHIFT/CTRL/ALT, since keyboards allow you to press more buttons at the same time if they include one or more of these three e.g. I use CTRL for L, and that's enough for it to work for me. It's pressing lots of keys at the same time from the letters and numbers and symbols that tend to not work.