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Over tightened primary can be one factor as Lee mentioned.
Did you also remove and clean the secondary mirror? If you did, and did not replace it with the correct orientation to the focuser, the laser can actually give you a false reading that everything is "good", when they are not. A laser alone will not help to collimate the secondary. If the secondary is not square with the focuser, it is still possible to get the laser to point to the center spot on the primary and tweak it, but when you look through the scope the image will be stuffed. To deal with the secondary mirror you need to use a Cheshire eyepiece, and use it to get the secondary mirror set correctly and only then ook at the primary.
Are you finding that you cannot get the focuser to ruck in or out sufficiently to focus? Like Lee asked, do the stars come to pinpoints?
Is your dob a solid tube or collapsible one? If the latter, one of the strutts may not have been fully extended or tightened - I've done this too and it will affect focusing performance.
Might seem like a dumb question, but have you looked down the draw tube of the focuser?
Alex.
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