Is the bathtub off level, or is the surround crooked?

Caulk is not waterproof so it may have come away from the wall during bathing etc even under steamy conditions. I would have used a proprietry filler. However, to see if it's level use a long level or fill the bath to a normal bathing level and measure the distance from the top of the water to the top of the bath. The measurement should be the same all the way around as water finds it's own even level. If the tub is not level you can do this by altering the feet below the bath as the bath stands on a metal stand or four metal feet which can be adjusted. If you are tiling then if the tiles do not fill in the gap between the wall and the bath skim this area with waterproof filler or plaster but make sure that they are level and even as tiles don't bend.