wont start below half tank?

hey guys  my little sister has a 2000 s-10 with a 4cylinder 5spd.  when she got it.... it wouldnt start if it had below a quarter tank of gas. now it wont start at 3/8 of a tank...   she has to keep the truck at a 1/2 tank or above in order for it to start.  the truck will run fine all the way to empty.... just not start.  the truck was a total but other than that problem it runs and drives perfectly.    has anyone had this problem or know what it is?   any imput is appreciated....  thanks much

I assume checking the basics have been done, i.e. fuel lines, filter, etc?  small debris may be settling,  is there any other perf issues like when gassing it or idle variations?

Sounds like the vent on the gas tank is broke or missing.

i have not checked the fuel filter i will do that. and no it runs and drives normaly.  i thought it may have something to do with the fuel pump but i dont know.  i will drop the tank and see about that vent line.