In a mesh network, which includes both the Z wave and zigbee that smartthings uses, any mains powered device can act as a “repeater” passing along messages for other devices using the same protocol. So your Osram light bulbs should all pass along messages for each other. You will probably need one about every 40 to 50 feet. So most likely all you’ll need to do is move a bulb or two into the middle of your chain. 
The Following FAQ will explain (this is a clickable link)