A number(only about 3) of people that try to log on my game are saying when they try to log on it says connection failed.

Couple of key points that might be worth mentioning.

- They CAN log on other games no problem.
- They aren't banned.
- I'm not the host.
- They were able to log on the game if I was hosting.
- It just says connecting...connection failed at the splash screen after clicking link.
- One player was able to join once, but now unable to.

Things that were tried. (Most of this stuff was done just to rule it out.)

- Link was wrong? Gave them a link to click which worked for me.
- Out of date Byond version? They used the exact version I was using.
- Turning off all firewalls.

Any ideas on what could be causing this?
Thought of it being the host, but if it is what could cause this so I can maybe do it to my computer and re-host the game and see if the same thing happens.