Homebrew PCBs

I have recently used some positive resist boards from eBay and am expereinceing 
exposure times of up to twice as long as the times it is taking for the positive 
resist boards bought from Circuit Specialists.  


Is this because they are old or defective?  

I have a UV exposure box and the eBay boards take 150 seconds and the new 
Circuit Specialists boards only take 80 seconds?  


Please tell me what you think?

Thanks, Tom