The pulpal floor needs to flat in order to better resist occlusal forces. Take a look at the work on this video as a reference:

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ERRORS

1. Did not constantly maintain the height of the bur while preparing across a path; The established initial depth was lost as the operator performed the occlusal aspect of the prep.
2. Lifting the bur as the operator tries to expand the isthmus width, thus creating steps or sloping of the floor along the bucco-pulpal/linguo-pulpal line angles.
3. Did not effectively utilize the shape of the 330 bur. Or did not use #56 to flatten the floor.

TROUBLESHOOTING

#330 bur is pear shaped, with a slightly flat tip and rounded corners, use this to create a flat floor but keeping in mind to maintain the height of the bur (or do not move occlusally) as you extend buccal-lingually. When used improperly, it is easy for a 330 bur to make a scooped out or concave floor.

May use #56 bur but avoiding the line angles, buccal-lingual walls, and cavosurface margins. Only then that the width of the cavity can accommodate the size of #56, before you can use it to flat the floors.