The Garmin fēnix has some trouble finding satellites: it can take some time to get a satellite fix. Recently the fēnix has gotton a firmware update that supports the "Extended Prediction Orbit" technique. The idea behind it (that works on other devices like the Forerunner 220/620 too) is that the GPS can find the satellites faster using recent information about the position of the satellites.
It works very well; instead of minutes it takes just a few seconds to get a fix.
The information about the satellites needs to be updated regularly because it is valid for seven days max. This can be done by using Garmin Express or Garmin Connect Mobile for iOS, but if you don't have these at hand or if you don't want to use them, it can be done by using the tool on this page too.
The satellite data comes in a file named EPO.BIN; on the fenix this has to be placed in the folder Garmin/GPS. On the Forerunner 220 it has to be placed in the folder Garmin/REMOTESW.