CiviCRM 5.13.1 API v3 Merge not working
Normally contact merge process get skipped when there is something common in both contact and merge process unable to resolve it.

e.g. 
address conflicts (same address type on both contacts).

on actual merge contact UI, we have option to override the record or add additional record with same address type. 

You need to pass Merge Mode in api call.
string mode: "safe" skips the merge if there are no conflicts. Does a force merge otherwise.

more info at Is there any documentation for &#39;safe&#39; and &#39;aggressive&#39; merge modes?