We can’t recall ever seeing a case where events don’t play for no reason. Have you tried @mathew’s suggestion of connecting the profiler to see if that sheds any light on what’s happening?
There are features of the UE4 integration which could play a part in making a playing event inaudible. The Occlusion Parameter, Ambient Volume Parameter and Ambient LPF Parameter settings in the UE4 project settings will automatically drive the values of corresponding FMOD Studio event parameters, which may in turn affect the audibility of events. These effects would occur for 2D as well as 3D events. You should be able to observe any such effects in the profiler.
If the profiler doesn’t reveal anything useful you could try to create a minimal reproduction of the issue by modifying one of our API examples to load and play the same events. It would help to narrow down whether the issue is likely to be related to using FMOD from UE4 and if the issue can be reproduced outside UE4 it is often much quicker to investigate that way.