If the beans are under roasted they will taste overly acidic as opposed to *bitter.
Can you take some good quality photos of the beans it might help us diognose?
Assuming that the bitterness is not coming from roasting the beans too dark? There are a number of things that could be causing bitterness.
Tipping/scorching is where the heat is too concentrated too fast on the outside of the bean. This is evident in an uneven color on the surface of the bean especially on the ends/tips where it will be darker. *I would recommend moving the heat gun back a tad. Better mixing will also help.
Uneven roasting will also result in bitter flavours Bitterness is a very powerful flavor and even just a couple of burnt beans will carry through in the cup. If you are getting a few darker beans in your roast then I recommend pulling them out before ginding and also look at better agitation for your roaster.
How dark you are roasting? Try pulling your roasts at the first signs of second crack rather then rolling second crack. Oily beans usually indicate you have roasted too deep and often indicate bitterness.
Im not sure how fresh your drinking your coffee but a few extra days rest can work wonders in mellowing out coffee that is otherwise too acidic or too bitter.
Where were you getting your beans previously? Im curious what coffee you were using before that you are now comparing your own roasts too.