There's no one simple answer to your popup question but I can give some general info. Basically, the popup system is designed for the occasional interaction between members and typically things such as responding to comments and so forth. In general, popups should be sent and arrive almost instantly. However, there are some circumstances that can result in delays happening.
One big concern is that we let spammers get on the site because as things stand they could create havoc via the popup system on here. We do a pretty good job of heading them off at the pass so to speak before they even get on but we need to have in place some precautions just in case one slips through the net. Consequently, I have an automated script which attempts to determine if popup messages are being sent as spam. It's specific to the LSF but probably has some elements in common with the spam checking done by the various web-based email services. What happens if a message gets detected as possible spam is that it gets temporarily dumped into a holding queue and at some time in the future another process runs which either lets it through or zaps it. Someone whose messages have been flagged in this way a number of times will have all their subsequent popups automatically dumped in the holding queue (until we manually remove the possible spammer status from their account).
In much the same way as the web-based email services operate, occasionally some messages are going to get flagged as possible spam when they are not but it shouldn't happen too often. Without disclosing the finer points of the checks that are made 2 things to avoid are: sending the same URL to a large number of people, or, sending a whole bunch of short popups in a very short space of time. The popup system is not intended to be used for real time interaction which is best done either in the chat room or via one's own choice of messenger program.
Don't know if that helps any, I can tell you that your own account isn't flagged for spam checks but it could be that whoever you're having exchanges with is flagged in that way. Bet you're sorry you asked now...
