Voyager made a few jumps between mid-Season 4 & "Ashes to Ashes"- 300ly in "Hope & Fear", 10,000ly in "Timeless", 20,000ly in "Dark Frontier", and "3 years" in "The Voyager Conspiracy", so the trail would suddenly go cold and only pick up a very long time later (that's 30,300ly + 5 years of their average annual traveling distance). Then there's the issue of there being 2 trails for her to follow for a time, trails which both extend over 10s of thousands of light years.Voyager wasn't following a bee line course back to the alpha quadrant, they were zig-zagging around investigating interesting things off their base course, this would have slowed them down.
In terms of tracking Voyager, they were leaving a clear trail of both do-gooding and destruction behind themselves. It wouldn't be that hard to figure out where the ship had been.
So as long as you had a fast enough ship, catching Voyager - no problem.
Also, something to remember is despite veering off for investigating things or avoiding problem areas, they would obviously try to get back on course. We always see the interesting times (episodes), not the bland times where nothing much happens and they just keep cruising through space. If they didn't try to get back on the direct route or something parallel to it, they would gradually deviate more and more from where they are going. It's not like they keep wandering; it's go off to the side, then go on a tangent to connect back to the direct route.
"Ashes to Ashes" is one of the geographic mysteries (bad writing, but fun to try and figure out. Nice episode though). In "Course: Oblivion", they were ahead of Voyager because they got some enhanced warp drive breakthrough, the irony being it emitted radiation that harmed Silverbloods, but not humanoids. Had they survived to reach Voyager, Voyager would have been able to get home a lot faster.1 - how did Lindsey Ballard catch up with Voyager - she'd be shot into space then re-animated then lived on a planet for 2 years then left the planet in search of Voyager and in the space of 6 months had caught up with them - so to recap, Voyager had a 2 and a half years head start yet she still caught up with them - what the hell was Voyager doing that slowed them down so much (in course oblivion, the copies were also ahead of Voyager!!)