I have a sliding patio screen door that doesn't seal completely at the
top. It's in the track correctly but there is a noticeable gap. Any
suggestions as to how to seal this to keep the bugs out, but still
allow opening and closing? Also, how can I make it roll easier
(already replaced the rollers)? Thanks for any help.

Is it not closing because the screen panel is warped, or because
the thing doesn't fit up against the house? If the former, a wooden
rod inserted in the fram should force it square, if the latter,
get some of that weather-stripping that's like a bristle-broom
in a metal channel, and attach that to fill/cover the gap.
It can attach to either the door or the frame, whichever looks easier,
and brush the other.