I'm on season 6 of a Voyager re-watch. In S6E20 "Good Shepherd," the Delta Flyer is sent off to investigate a "class-T cluster" that's worth a look but not worth altering Voyager's course. This is just one example--the DF and other shuttles are often deviating from Voyager's course to carry out some other mission.
Voyager usually doesn't just stop and wait for the shuttles to catch up.** Also, Voyager is travelling in a fairly straight line, so the shuttles can't take a shortcut (if there was a shortcut, Voyager itself would just go that way). And the regular shuttles can't match Voyager's speed.
So how are the Delta Flyer and other shuttles able to go on side missions and then rendezvous later with Voyager?
*It does happen occasionally. In "Tsunkatse" (S6E15), Seven and Tuvok have plans to take a shuttle for an astronomical survey while Voyager orbits a planet for shore leave.