Use of dark slide

I have a new lens for my Bronica SQA. It's a 150mm but in some shots I think

that the film take light (the negative is completly dark). Anyone know if I have

to use the dark slide when I change lens, because apparently in 12 esposure I

have only 5 correct photos and some are missed when I insert the 150mm.

You do not have to use the darkslide when you change lenses. But with Bronicas the shutter is in the lens and if you have a problem with a shutter it will mean that the camera works OK with one lens and not at all (or spasmodically) with another.

You really do need to work out whether the problem exists with the 150 lens and only that lens. There are other potential explanations if that isn't the case.