It is going to take some time to get used to using a 60W laser. Cutting is no problem, power settings were easy enough to figure out. But engraving is a little harder. 20% power at 350mm/s is about the highest power I can run to get nice engravings on plywood and mdf. Photos are my hardest one to figure out right now. I guess this just gives me an excuse to play with the laser more. 
Well that’s the complexity of engraving. It’s all about the tube’s characteristics. I assume that a high powered tube might have more problems in managing low power ranges. I might not even go below a certain grayscale but just switches off. I found the 40 watt already very challenging to produce nice grayscales.(2 mA range)
On the plus side it blows through 6mm plywood or mdf like a hot knife through butter.
Try slower and lower power - you want to scorch the wood, not vaporize it.