@Paultreefrog,

The only thing that you can really do to adjust the signal strength from deco to deco is to move the unit. 

Make sure that the Deco is not covered, under a desk, or next to devices that could cause wireless interference. Sometimes, even the corner of a wall can cause a signal to degrade quickly. 

If the signal remains weak, you may have placed the satellite unit too far away from the main node. You can either add an additional node between the two, or move the satellite closer to the main node. You may want to look into having a wired connection between your nodes, as this provides the best connection and stability to the network.