sounds like it needs adjusting. When properly adjusted
1) with the slide IN, it should seal completely on the sides and top on the outside with the "bulb" seal commonly used. Ideally it should be uniform top to bottom.
2) with the slide OUT, the bulb seal on the inside should also seal against the trim.
In addition, slides can usually be adjusted to move the slide side to side and up and down.
with adjustment points it can usually be adjusted for tilt as well. And if one side is in/out more than the other, that can be adjusted for both the IN and OUT position.
can it be adjusted as a DIY??? looking at the instructions for mine I'd say yes if you also can visualize and understand how changing on adjustment is also going to affect the other adjustment.
with the gap your seeing, vermin aren't the only issues. WATER is the other one I'd be concerned about.