This might be a bug, report
here. But I do have some tips
- Check if they have instant messages enabled. This can be disabled in chat settings
- Check for misspelled names
- Use the /msg or /fmsg command and not /tell
- If you’re playing without mods, try to play in the latest version of Minecraft
This seems to, if it’s a bug, a Cubecraft related bug and not from Minecraft itself so I don’t know enough to give a definitief answer.
When making the bug report
- Have enough and clear evidence
- Try to find a way to reproduce the bug
- Use /whereami beforehand
You can find tips on how to record your gameplay
here, it is aimed towards hacker but can be used for bugs as well.
I hope this helped,
~Fesa