There can be many reasons. Pure white crystal DMT, left out for a long time, can turn into DMT n-oxide, which is a yellow oil. If half your sample turned into this yellow oil, it would be gooey, but not poorly extracted nor have much of anything that is undesirable in it. It could also be plant fats, which would give a yellowish brown gooey texture and a harsh smoke.
If you spend a little time cruising the site you should be able to find a number of methods to take whatever you purchased and purify it. Or, even better, you could extract it yourself. That way you know what you are putting in to your body.