Recruitment is usually the longest phase of a trial and the one most likely to overrun. Because the trial cannot finish until enough participants have been enrolled and followed, a slow recruitment phase pushes back everything after it.
Why the search is inherently slow
Eligibility criteria are the rules that decide who may join. Each rule removes people from the pool, and the rules combine, so the number of people who satisfy all of them at once is much smaller than the number who satisfy any one of them. A trial therefore has to look through a large population to find a small set of matches, and that looking is done by people reading records one at a time.
The cost is not linear. A trial that enrolls at half the planned rate does not simply finish a little late; it delays the point at which the last participant completes follow-up, which is when analysis can start. Recruitment delay is inherited by every later stage.