Anthony F's answer says it all. Likely something is still animating. The system seems to wait for the app UI to "settle" (go idle) and when that happens, it performs the tap action. However, when the UI constantly runs animations, it will never settle.
Xcode Console Output
Enable the console output in Xcode to see what happens when running the test.
Below is an example of the log, when it works well. The system waits for the app to go idle and when that has happened, it goes to find the button in the hierarchy.
t = 16.95s Tap "@go" Button
t = 16.95s Wait for app to idle
t = 17.00s Find the "@go" Button
t = 17.00s Snapshot accessibility hierarchy for XXX
t = 17.09s Find: Descendants matching type Button
t = 17.09s Find: Elements matching predicate '"@go" IN identifiers'
t = 17.10s Wait for app to idle
t = 17.15s Synthesize event
t = 17.41s Wait for app to idle
Below is an example when it fails. The system waits for the app to settle so that it can look for the button in the hierarchy. Since, it does not settle, it waits "forever" finally running into the timeout.
t = 18.88s Set device orientation to Unknown
t = 18.93s Tap "@go" Button
t = 18.93s Wait for app to idle
t = 79.00s Assertion Failure: UI Testing Failure - App failed to quiesce within 60s
In my case the "failed to quiesce" was caused, because at time t=18.90s, demo data was generated which caused repeated updates of a UIProgressView. From then on the app UI never settled ("quiesced").
("quiesce" as a word is not in my active vocabulary, which certainly has delayed my recognition of what was going on. The console output pushed me into the right direction. I'd bet "idle" rings more bells than "quiesce" for many developers.)