I think until the tensioner is filled with oil you will get some slack on the chain between the cam sprockets when you rotate the engine due to the valves pressing on the cams. If you have slack on the part of the chain that runs up the tension rail then i'd check the tensioner. If it is the tensioner, try coating the inside of the tensioner in oil and install it already activated. Its a bit tricky but makes sure the tensioner is activated.