Players RPG Dependable
There are a few ways to "Play" An inconsistent player.
I don't allow a player to play who does not have a background to his/her character. Motivations, Obligations, Responsibilities, and Attitudes are all important. If the player is then unable to attend, I can "Sock puppet" Their actions and responses.
If in activity becomes frequent, I will notify the player that his character is going to be NPC'd and then I find another active individual.
In the Play By Post environment, having a means to send reminders for players' turns to the respective players is crucial. I play in several other Play By Post games where the community exists outside of the game. Between emails and social media, we are kept up to date on events related to the game and those playing the game.
As a GM, I do a lot of work in keeping things interesting. If a player is interested, they tend to be more motivated to be involved.
Some players are not cut out for Play By Post as there is not as much action as a FtF. But discerning which format fits which player is tough to do with players new to Play By Post or those new to RP in general.