I do not accept documents a few at a time. The client needs to get all documents together before giving them to me. I set deadlines for my clients. If their packet of documents is not received before 3/15 they are subject to extension. I file extensions for all returns I have not completed by early April. 

I do not file extensions for clients who do not respond to my asking on 3/15 whether they intend to use my services. This is when I ask whether they will require an extension (which they have to pay for before it is filed).

You need to train your clients to play by your rules or go elsewhere.