On Jun 5th, there were tasks pending in pool luci.flutter.try for almost 3 hours, causing test failure. This happened mainly on bots with OS: Mac-10.14.6. This further caused Engine build failure.

Active tasks and max pending time for Jun. 5th.

Bots (Mac-10.14.6) status over that time period

We can see from above charts that Mac bots were fully occupied for most of time of the day, and caused task pending/scheduling timeout.

Extra bots seem to be needed.