What other components are you using in this? For instance, if you're using a plastic straw as a mouth piece there is a good chance that it is too far into the light bulb and thus melts causing you to inhale plastic vapours. Surgical tubing instead may help, if it is available to you. Other more suitable materials may be found on the web.

I never found light bulb vapes to be very efficient with herb at all though, crack and heroin is another story.