Does your darkslide have notches? If so, then the interlocks that keep the darkslide from coming out are dirty and cannot engage the notches in the darkslide... I had one back do this, I cleaned it with a bit of non-residue contact cleaner and some compressed air. I'm at work, so unable to give pics

If your darkslide is older and has no notches, that could also be the issue, since that feature started with the Pro-S IIRC.

Good luck!