If you don't have the recoil spring on right, the slid will not go back on the gun. First, make sure you don't have it in backwards!! Quoting from the manual: "Note that the recoil spring has one wide balck end and one narrow black end. Fit the narrow end against the forward part of the slide and compress the spring slightly. Fit the wide end against the barrel lug. There are two steps on the lug. It goes against the front step, which has the tapered ramp leading up to it.
Carefully line up the rails on both sides of the frame with the groves on the slide. Fit the two units together, and pull the slide back until the takedown catch clicks into place."
It's sometimes helpful to use your thumb to compress the spring in while putting the slide on.
Also, make sure the striker unit is on right.
" Put the dococker in it's slot, press it all the way down and hold it there. Slide the striker unit into the striker tunnel and push it all the way in. Slide the retaining plate into it's slot. When you push it all the way up, you will hear it snap into position. If you have done it correctly, the decocker will be retained in the slide by the striker unit.
I'm sure this is a case of you not having something lined up right!!
It may be helpful to dissemble the slide and start the assembly process over, following the above directions starting with the striker unit first!!
If all the above is correct, try pushing on the ejector as you are putting on the slide.
If I remember right, I had the same problem, years ago, when I was in the P99 assembly learning curve!!
