Make sure that you have all the datacrons for an achievement on one character. The achievement does not track the datacrons across characters. Also, make sure to log in on that character.
Nope that's completely wrong, it most certainly is tracked across all characters. It doesn't matter which toons that you have unlocked which datacrons on, they all should count toward the planetary total. Beastmaster, Loremaster, Galactic Explorer, any of the ops or PvP achievement are all the same, do them on any toon and they should be updating toward the whole.
There are a number of achievements that seem to be bugged retroactively, though most I've seen reported are items.