Cannot duplex print - Macbook OS 10.6.8 with Dell 2335dn

I cannot print 2 sided from a Macbook running OS 10.6.8. Have reset the printer settings and deleted and re-installed the driver. Have downloaded and installed Mac OS X driver version A06 without success.

Have no trouble printing 2 sided from a different Macbook Pro running OS 10.8 over the same network.

Suggestions?