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There may be also a misalignment of the focuser too. You can "achieve" collimation just by tweeking the secondary and primary, but the focuser is often overlooked, resulting in still unsatisfactory collimation. Frustrating if you've dumped a whole pile of cash on a new tool.
I've had a little issue with collimation too. No mater how well "collimated" my 17.5" was, I always had a tiny tail come off the brighter stars. I've also tried different types of colliamtion tools, yet they all gave the same result. If there was a problem with the primary, this would have been noticed in variations to the image across the FOV in and out of focus, but the orientation of these tails was unilateral.
I then decided to check my focuser. Using nothing more than an old Kodak film canister, I made a little hole in the centre of its lid & bottom. I also marked the centre of the secondary, with its off-set that I've built into the scope. I stuck this canister in the 1.25" size eyepiece hole, and looked through the holes to see where the secondary was. Guess what? The focuser was off alignment by a tiny amount. I'd say two degrees. That was all that it was. The focuser's alignment was adjusted, and those little tails have finally disappeared.
The focuser is often never suspected as a collimation issue. Sab has suggested this too.
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