You never want the workpieces to shift while you’re routing the joint. So it pays to make sure they’re held firmly beneath the clamping bars. If your jig doesn’t have grip tape on the clamping bars, consider adding some adhesive-backed sandpaper to create a non-skid surface. And when clamping narrow parts, I place a spacer under the clamping bars to keep them from racking.
The fit between the pins and tails is determined by the bit height in the router. This is your main adjustment when setting up to cut the dovetails. So the first thing I do is adjust the bit height with test cuts using stock identical in thickness to that of the drawer parts. I rely on a simple verse to help me remember which way to go, “Lower to loosen — heighten to tighten.”
It’s always hard to know when the fit is just right. I shoot for a joint that where the tails fit halfway into the sockets by hand. Then a few light taps with a mallet should fully seat the tails.