If your straps are adjusted to the right length and are still falling off, then it may be a larger fit issue beyond just the straps. The first problem could fall within the cup size. The most common cup issue that accompanies falling bra straps is a gaping cup. If you notice a space between the top of your cup and your breast then going down in cup size could be the solution to your falling straps.
On the other hand, the more common fit issue that causes the bra straps to fall is wearing an incorrect band size. Bra straps are set wider in the back in larger band sizes to accommodate larger busts. As the band size decreases, the straps are set closer together in the back for smaller busts. If the straps are too far apart, then they will be prone to falling off. Try going down in a band size to help keep the bra straps from falling.
Don’t forget that as you adjust band size, the cup size will need to be adjusted as well. For example, if you decrease your band size the cup size increases, as in going from a 36C to a 34D. Keeping this in mind will ensure you keep the same cup volume.