do you shake your leg? like when you're reading a book, or reading newspaper, or when you cross leg? if you do and the 'shake' is in your left leg then you're lucky. cos when you play your left leg will auto turn on. too bad for me mine's in my right leg, and it's hard to keep time (even though when my bass is not playing my right leg will tap) because my right leg has quite a lot of work to do. but yeah left-leg-tapping CAN be trained; when you practise rudiments (i assume you do, if not, just whenever you practise) bear in mind to shake your left leg instead of your right, and if you don't even usually shake your leg, force yourself to. it really helps. if you can't coordinate both left and right legs, firstly practise, and secondly try different common beats with left-leg-tapping. you'll find, in the same way when you practise a bunch of different beats and that makes you be able to play any other beat, when you practise left-leg-tapping (honestly, is there a proper term for this??) for a few beats after awhile it becomes natural. see Chad Smith. NO MATTER what he's playing his left leg is forever on.
cheers.