Hi there!
Thanks for reaching out to us in the forums for this issue. Sorry to hear that you're unable to connect to a buddy in Minecraft. Its possible that you're unable to connect with your other friend due to a NAT type issue on their end, since you've successfully
been able to connect with others on your end. However, you did say that there are already 4 others in there with you and with Minecraft: Windows 10 edition, you'd be limited to playing with 4 others online (or locally) unless you are subscribers of Minecraft
Realms (this allows up to 10).
For a bit more info regarding that, I recommend the following articles:
Minecraft: Windows 10 Edition FAQ
Minecraft RealmsMinecraft
I hope that this helps out!