Run through this checklist, from most likely to least likely scenario:

1. Try tiling your texture in the graphics program. If it tiles properly in there, continue on. If not, fix this.
2. Make sure you don't break up larger blocks into several pieces as this could cause this. If the blocks aren't broken up, continue on. If not, make some test block off the side somewhere so that the texture can tile properly to see if this is the cause.
3. Check to make sure there's no ent_created entity there or something like that or that you don't have blocks or other objects intersecting each other.

Beyond here, I'd need more details. What version and edition do you have?