ceiling staining

we put a metal roof on our manufactored home 5 yrs ago. we started noticing staining in the ceiling about a yr after the roof was installed . we called the roofers and they said it wasn't being caused by the roof. i asked if the roof was properly vented . they told me yes. they put a ridge vent on the house . the staining is still happening and actually getting worse. we had the roofers come out and checked everything and they said the roof is not leaking. i asked them again if the roof was venting properly . they said yes. i did some research and from what i found . they said if staining is happening on the ceiling the roof is not venting properly and moisture is getting trapped in the insulation. i asked the roofing if baffels should of been installed over the insulation. they said no. my question is what do i need todo to stop the staining in the celing and do i need to have additional vents added to the roof . i can send pictures of the staining if you would like to see it .