Check the 3 rubber seals in the radiator cap, along with the surfaces in the radiator neck where they're supposed to seal against. Nicks in the radiator neck let it suck air instead of sucking coolant back in.

Same story with the hose going over to the reservoir. Ever try drinking thru a straw with holes in it. You get air instead of your drink. Check that hose, & the funky connections at the reservoir cap, & the hose that's supposed to go down to the bottom of the reservoir.

Finally, if all that stuff is OK, maybe you have to worry about your headgasket?