There is no general issue with Scheduling, that is for sure, in which case I'm not certain if it's something that we can 'hotfix' for you, as that seems more like an environment issue and Xibo issue.
If 'always' works but custom times does not, then there must be discrepancy between timezones in CMS, server and players.
Similar if scheduling with 'run at CMS time' and without it gives different results then it's clear that the timezones do not match.
You can enable auditing on the player as well as assign a display profile with audit log level to it.
Then in logs you should see schedule.xml file for this player, which should show what exactly it 'thinks' it should display and when.
If similar problem is on windows client, then schedule.xml is in the player local library if you have access to the PC running this player you can get it from there.
Of course if you want me to look at those logs or schedule.xml, please do send that to me via private message.