Cups -- not printing some gray tones

testimpression.pdf (20.4 KB)
In this PDF i put some text in different gray tones... when printing it only shows the first two...
It is very annoying I print some documents and a lot of informations are not printed because they are in some tones of gray :{

Any idea how to fix it ?
I will try to print a test page from HP tools (web service) this printer is wireless to see if the gray scales are printed as cups test page that seems correct I can't compare because I basketted the previous test pages.