its probably because you have a few double vertices. If you select all your verts and go to the edit vertices>weld settings button. the one next to weld in editable poly and make the weld threshold something very low. usually the default works if vertices are very close together. Then hit apply or ok. doing that is good when you may have several problems and you want to fix them all quickly but if the threshold is too high you can accidentally weld things you didn't mean to weld. Alternatively you can select the troubling vertex and check if you have more than one selected under your edit poly. then just hit ctrl+alt+C to collapse the two. Additionally it is really use full to use the symmetry modifier which welds the seam automatically. it takes some getting used to though.

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Cool thanks for the tips. I'll try these out

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