Jay's oops has me wondering now. Did you mount a standard ceiling box so it is parallel with the pitched ceiling or floor? The box should be mounted into the ceiling flat/parallel with the ceiling. You then need a ceiling fan extension rod that has a ball and socket type swivel adapter the screws into the box. The flange/cover plate is designed to fit over the ball and socket assembly and fasten and go against the pitched and flat ceiling as long as the pitch angle is not to great. If you don't have a rod with swivel then you need to buy one.
Your post almost made it sound like you installed a box parallel with the floor so it sticks out of the ceiling at an angle which is a no-no.
The pancake box would only be used if you have a shallow depth above the ceiling where a standard box won't fit however the rafters should be of sufficient size to allow a standard box.
Post back with what you got and what you did.
By the way, Jay's leftover screws are probably are the ones I get shorted everytime something comes unassembled. Murphys law, everytime you get shorted a screw or bolt, it will be either metric, an off color, an off size, an off length, an off thread, and definitely something not carried at a hardware store. LOL