i had this when i first started mining there. it is rare items gem stones. i spoke to a experienced miner and this is the tip i got that works almost every time : when you bar is so small it is a rare item, think of your target as percentages (the round meter that you press on to strike) so the outside is 100 percent and the middle is 0percent, to mine the rare items you want to strike at about 25 percent. and the ore and alumin is at 75 percent.

i hope that is clear and you understand what i mean. once i got it i hardly ever miss anything