What is the deal?!

ok so i have a 1980 ford f100 manual 6cylinder. i just bought a new battery because the truck was always dying on me at a light for instance and if i left it over night it wouldnt start the next day without a jump. with the new battery it starts right up. i also just got new spark plugs. anyway my question is i started it up today. and it was fine. i then turned it off and went inside and when i came back outside to leave all the sudden it doesnt turn on anymore. it cranks fine but it wil not start. i put more gas in it and nothing. the battery terminals are brand new. im pretty sure i checked all the connections right. can anyone help me?