Complex sentence: Oldham and Ipswich blew their chances in the semi-finals, while Leeds were defeated by Charlton in the final to miss out on promotion--an FA Cup semi-final defeat had ended their chances of success in the cup competitions.
Split into simpler sentences: Oldham and Ipswich blew their chances in the semi-finals, while Leeds were defeated by Charlton in the final to miss out on promotion. An FA Cup semi-final defeat had ended their chances of success in the cup competitions.