Hoisin sauce is naturally a thick sauce because of the ingredients used to make it.  If you prefer it to be a little more runny, like a pouring sauce, you could let it down (make it thinner) with a little more water (perhaps another tablespoon or two).  Conversely, if this sauce seems too thin, you could thicken it up with some corn starch mixed with a touch of water.  You would have to whisk this into the sauce while cooking and then heat for a few minutes to cook out the flour flavour.  I find this unnecessary though as the ingredients I use tend to give quite a thick dolloping consistency.  It also thickens a little in the fridge so be careful not to thicken it too much!