Need more information. Numerous reasons:
Your delay is not long enough on the replay timer
Your delay on the replay timer is set to 0, and the pixel capture didn't work right.
Your resolution settings are wrong, causing the mouse to miss the button
The computer is screwing you with lag/connection errors/not letting the bot work at potential.
The server is giving you errors like Connection Timed out, preventing you from clicking anything.
More information regarding settings?