But... is this normal? Do most games fail to gel? Have I just been unlucky?
It happens more often than not.
It's just intrinsically hard to run a RPG in PbP mode, because you can't interact with the other people in real time. Most people can't stand waiting, it becomes a torture, and then they drop out. I've played a few PbP over the years, and most of them died during the introduction or within the first encounter. Too few games lasted long enough (IIRC I played in 3 games which lasted ~1 year, equivalent to 2-3 encounters) to give me a clear picture of how to make a PbP game work. The only one which lasted longer than that was the game which I've run myself, ~2years for 5 encounters, then I had to pull out myself because I had become unable to post frequently enough to keep it interesting.
One suggestion which I dare to give is
keep it simple. IMHO most of the times both the DM and players start a PbP game with too much initial effort and expectations, putting too much at stakes too soon and quickly burning out.
These are some examples of behaviours which would be awesome but can also kill a PbP before it really starts:
1- the DM writing too much about her homebrew setting
As much as I love homebrewing, none of the players will ever be interested in my setting as much as me, and they will keep forgetting what I write about it. It's better if the DM goes light on the setting, for example limit herrself to a few lines to describe only the big deals about your settings, or don't do even that and just start the adventure locally, let the setting reveal itself gradually. Another alternative is to use a popular published setting, and let the players decide how much they want to read about it on their own.
2- the DM using lots of house rules
Most of the times house rules don't make a difference in PbP, but unfortunately there are DMs who specifically look for PbP as a way to revel in their own house ruling, which maybe they can't do when playing tabletop. But those HR don't make the PbP game better for the players, and instead they slow them down already during character creation.
3- the players writing too much character background
Plenty of background details and character descriptions are great if you're going to play the character for a very long time. In PbP you'd better assume you won't, and it will be just wasted time. The DM herself should never require the players to write a detailed character background in a PbP.
4- the players micro-managing their character sheet
Fiddly bits lead to errors and are big time-wasters, and the worst part of a character sheet in my opinion is the equipment. It's better to encourage people to use ready options like starting packages or standard arrays, not to give them too much freedom with equipment, or at least be approximate about starting money. It's insane to make precise cost calculations when it's more probable you won't have time to use most your equipment, and to spend the treasure in new stuff. Much easier to just tell everyone something like "pick 2 weapons and a common magic item".
5- the players roleplaying too extensively
This is supposed to be the bright side of PbP, that since you can think more carefully about your character's speech, and you don't have to do it personally in front of others, written roleplay can be more satisfying for a lot of players. But be wary that other players can become annoyed if they don't have much to say and have to wade through pages of roleplay waiting for some action.
Bottom line:
The biggest threats are at the beginning, so it's better to do all you can to make the beginning phase as short as possible. Just consider that many players join a PbP game
when they feel inspired to play some game of D&D. If it takes weeks or even months before the game really starts, chances are that some of those players have lost momentum already.
