Flying in Solo I go to a Low temperature hot spot in an icy ring. I have a lot of experience mining so I slowly find 15 asteroids with fissures. I fire a prospector limpet at each one and not one of them has diamonds. Opals, Bromellite, Alexandrite but not one diamond asteroid. I mean 15 asteroids in a row. What are the odds? It reminded me of the movie "Jurassic Park" when Dr. Malcolm asks "Now you are planning on having dinosaurs on your dinosaur tour, right?" There are supposed to be diamonds in the low temperature diamond hot spot, right?
Yes, but not in the fissure asteroids.
They are in normal ones and you mine them by normal mining lasers.
Opals are great, too if you prefer core mining, though.
Low-Temp Diamonds show up in all forms(Regular, Surface, Sub-surface, Core), might have just been a bad run
Single hotpots don't change the odds by much, there's a reason the triple at Borann is so popular.
Dude need some help !!! cuz my collector drone collect everything but LT Diamonds (even on 20% and more asteroids) i dont know what i did since i change my ship !!
sounds like you have added LTD to your ignore list.
The next time you go mining, check a LTD chunk in the Left Hand Panel and see whether it is active or on your ignore list.
OMG thanks Man i doesnt know that...its been a week that ive start playing it.... So much to learn !! Ill let you know 
An LTD hotspot will increase the chance of fnding more LTD cores in that ring than other cores. But not by very much.
I mean, my best worse streak was 5 bromelite cores, 1 alexandrite and 2 ltd cores - all those in a Void Opal hotspot.
For better chance of getting a core is to try the overlapping area of a double hotspot.
And i heard that at Borann - one will find ONLY ltd cores in the overlapping area.
No idea tho - i never verified that personally, but in all my laser mining session at Borann, i randomly stumbled over cores and they always had ltd (the sample was small enogh to treat it as a simple coincidence - 4-5 instances)
OP, the exact same thing happened to me yesterday, fired 18 prospector limpets in a row, nothing! I think its possible we both entered the asteroid field at a time before the rocks replenished themselves.
Adding to what @Snowymonk said, be sure not to add engineering raw materials to your ignore list. Occasionally an asteroid will drop iron, carbon etc. You want to collect these to use for engineering, which is fine as they take up no cargo space.
When I first started mining I added everything but LTDs to the ignore list, not knowing the difference. Missed out on a whole bunch of raw mats.
Until you’re full - and then ignore because it wastes precious limpet time. Just have to manage that carefully