So, this has been happening again now that the network is getting more activity.
There is an issue with Second Life where an object will stop getting e-mails. This issue is at the sim level, not the scripts inside. Until the sim is restarted, the object will not receive most emails sent to it.
Unfortunately, the ASN was built before http-in existed, so the gates rely on emails via the AFVIs to establish a connection between gates. This means that if your gate gets hit by this bug, you will not be able to dial it from another gate until the sim your gate is in is restarted.
If you are on a region you own or rent, this can be done fairly easily. For those of us on mainland, this is nearly impossible.
Until I have a significant amount of time to spend on this project, I will not be able to build a 1.2 release which would add http-in to each and every gate directly (thus eliminating the need for the AFVIs).
So, for now, if your gate stops getting incoming connections, have your sim rebooted or wait for the rolling restart of the grid.