There's a lot of factors that can work in concert to create delays in PBP's I've found. By far the biggest one in my experience is when there's an open decision before the party (that is, a decision that is not quantified, like: Where do you go now? or What do you do next?), as opposed to a closed decision, which is where there's a limited number of obvious choices like, which of the three buttons do you press, or which of the two doors do you open?
There's this snowballing thing where people who think they -might- have an idea pause to see if anyone else posts who has a -better- idea. It only takes a few days to get out of the flow and get out of the 'habit' of posting, so to speak.
This -will- happen from time to time. I've never seen a game where it didn't. You can have the best group with the best GM, but sometimes you'll still get that 'shrug and go 'huh' moment. At a tabletop, it can slow things down for a few minutes. In a PBP...it takes longer. The only advice I can give is that sometimes the GM -does- need to kick the players in their collective patooties and get an answer.
As a player, waiting to see if someone else takes the lead is seductive. It leaves you blameless...an anonymous member of the crowd. After all, no one ELSE is posting either, right? It absolves you of responsibility. It promises to deliver blame and responsibility to the poor schmuck who posts first.
As a GM, breaking that up is one of the things you have to do to keep the game moving. Usually a reminder is enough. Perhaps with a veiled threat to NPC -everyone- should no one post soon.
One thing I heartily advise though is not to take it personally. It is in no way a commentary on your game, nor even on our commitment or enjoyment of your game. Like I said...it happens to every game sooner or later. That it's taken this long in your game is actually pretty remarkable.