Screws through WEDI panels are not long enough to reach stud spaced away from studs. What depth should screws penetrate stud? Longer SS screws.

If you are using the standard 1/2 .in board we need more information, something might be wrong. If you are using a larger thickness board you just need to get a longer exterior grade or other screw that's rated for concrete board. Deck screws are typically a good option.