3 month old 30 amp hour battery. Had no starting problems until recently. Battery shows 12.5 volts, barely cranks over and won't start. Lights and winch both work.
Put a voltmeter on the battery and watch it when you are cranking the engine; if it drops below 10.5 volts you may need a new battery. Did you charge the battery until it was full before or just after you installed it? Did you use a battery charger not a battery tender? Check the date on your battery, it may have sat on the shelf for a long time.