Could just be bad luck, I've had some unproductive days too. Even in the triple hotspot, LTD asteroids are in a minority, especially those with decent yields.
Make sure your ship can run at least 2 prospector limpets simultaneously (a size 3 controller or 2 size 1's). You can only target one at a time, but you can turn and fire another while the first is in transit. When the first hits and reports back, immediately target the second and turn to fire a third (if the first failed to find anything useful) until you find something. Always aim to have two prospectors in flight while searching. Begin with a hold almost full of limpets (I usually go with 250 in a 256t hold), better to have too many and dump some than not have enough.
While mining, your speed will probably be limited by your number of collector limpets rather than your lasers (my T10's 11 collectors can't keep up with 5 mining lances: I could mount size 2 lasers on most of those hardpoints for more blasting power, but I already have more than I need).
It usually takes me a couple of hours or so to fill my 256t hold.