This might be because it’s already taken, it’s on our naughty list or the combo’s just not possible.
Here's a few examples:
Consider using a 1 instead of an I or a 5 instead of an S. If that doesn't work, try a 3 in place of an E, or even a 4 as an A.
Keep in mind, some styles have certain combo rules. If you have a look in mind, start by checking out the styles first, to see which combo's work in that style.