NPC commands only triggered 10% of the time

While creating a quest on Bedrock, i summoned 11 NPCs, 9 of which are quest relevant and get teleported around a bit. I'm going to ignore the 2 npcs that just stand around. The quest depends on the NPCs to execute predefined commands upon player interaction. 7 of 9 npcs do so reliably, but npc 6 and 7 fail to operate their commands more than 10% of times, no matter which command is set: for example: /title @a title test

each command has been tested via commandblock and via chat console to eliminate errors of the command itself.

The question is: why does it not work and how do I fix it.

PS: uploading my world does not work as the file is almost 800 MB.