Granada,
Like Rick says, some laser collimators need to be collimated themselves!
Have a read through the following IIS thread I started about how to check your laser and how to collimate one of these:
Tricking up your GSO laser collimator
One thing that a laser won't do is actually tune the secondary mirror! A laser will not help with the centering of the secondary in the focuser, and won't check if the secondary is square with the focuser. This last point alone will mean that if you rely solely on the laser to "collimate" the secondary, if the secondary is not square then whatever you think your scope is collimated, it will actually be a false reading.
Many people think that "primary" and "secondary" also relate to the importance of each mirror. Not only is this incorrect, but the truth is the secondary mirror needs to be sorted out FIRST before the primary. And this means not just that the reflected laser beam is on the centre mark, but that it is also properly collimated in relation to the focuser and in the optical train. Only when the secondary is properly sorted in all its parameters, only then are you ready to deal with the primary.
I too fell into the trap of solely relying on a dumb laser for all my collimation protocol. And as time went on because the secondary was not being properly treated, the image was progressively getting worse and worse, despite all my efforts with the laser. It was only because I had a gut-full of a s.h.i.t.t.y image, and having exhausted everything I could do with the laser, for some reason I decided to check what was going on with the secondary. To my amazement, the secondary mirror was totally WAAAYYY off! Way out of center, not square (rotated) and off to one side. I was stunned because I thought I was doing everything correctly, but NO ONE had ever mentioned to me that a laser is only a tool to use AFTER the secondary is sorted, and that the laser won't deal with it, not even the bloody astro retailer I bought the blasted laser from.
The humble Cheshire eyepiece is the best tool to use to deal with the secondary. A Cheshire eyepiece will sort out the entire collimation process of a Newtonian, though a laser can be more convenient to deal with the primary. Once you have dealt with the secondary with the Cheshire, then you can use the laser to first fine tune the centering of the beam in the centre spot, and then you can tweak the primary.
So, you have to do two things, get your hands on a Cheshire eyepiece, and verify that your laser is properly tuned too which is very easy to do.
Alex.