One factor which can make a big difference to both volume and precision of hitting a string is the length of the fingernail. If the nail is too short, the finger tip below the nail will still make contact with the string. This deadens the tone and volume. It also provides less chance for a clean attack on the string from a point of view of accuracy also.
If the nail grows too long, it can also affect accuracy, and is also more likely to break.
Somewhere in between gives the best note.

For drop-thumb work. I usually find that I hold the thumb in a relaxed, but basically fixed position, and the thumb is brought into string contact by a slight twist or roll of the wrist. Occasionally, however, for specific patterns (when the 5th string does not fit into the pattern), a VERY rigid thumb alternates with the index finger on the strings.
Quack!
GtD.