People will need a whole lot more info as to diet, training, etc. I'd say if you are going for a burn that may be part of you problem since the burn usually means lactic acid buildup which can mean you are working in the endurance range rather than the strength building and/or muscle building range. I'm talking in vague generalities here because I have no idea what you are doing.
I'll have to state something obvious here: There is no way someone could tell you what you are doing wrong if you haven't told them what you are doing