Any number of things could make it hard to start. A couple things to check would be the condition of the reed petals, the carb settings (float level, pilot jet size, etc), compression of the top end, electrical connections (all the wire connectors and ground points- this includes the spark plug cap and wire going to the coil), and check for any air leaks.
Some bikes (CR 500's for instance) are a bit reluctant to start without having anything major wrong. Tipping the bike over until fuel runs out of the carb overflow hoses raises the float level temporarily and can make starting much easier.
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