I've just replaced the rear derailleur on my kids bike. I've fitted the biked with a Shimano Altus. The issue I am seeing is the shifting between gears 6 and 7. When I move from 7 to 6 the chain does not move onto the next cog. When I move from 6 to 5 it moves to the next cog and then continues up with each gear change. So effectively when I'm at 1, the chain is on the second last cog. When I shift from the gears from 1 to 7 the same behaviour is seen. So effectively when I move from gear 5 to 6, the chain has already moved onto the last cog. So when I move from 6 to 7 the chain just sits there.
What do I need to do to ensure that the chain moves from the smallest to the next cog when I move the gear from 7 to 6 ?