Various sections of IMDb are processed separately and on different schedules. Depending on various factors some contributions can take longer to appear than others.
Data is always tied to a title. Data items for a new title will only appear after the title has been accepted and added to IMDb. This explains why new titles can initially appear to be empty: their cast/crew credits will usually begin to show over the following days.
Credits for the same person can be updated at different times (i.e. two credits for John Smith on the same film, one as actor and the other as writer, might appear on the site at different times even if they were submitted simultaneously).
Names that were not previously in the database require additional processing that usually adds a delay. For example, if you submitted 15 cast members for a film: the credits belonging to people with existing filmographies in IMDb will appear before credits for new/unknown actors.