Without logs its going to be hard to analyse what is the particular issue.
It could be numerous things:
Talents
Gear Stats
Ability/Skill
And it is likely a mixture of all those, there are a lot of things that you can do to squeeze out more damage such as timing chaos bolt dumps with trinkets and buffs and efficient use of backdraft. General Tips would be to keep immolate up almost 100%, Never cap conflag charges or embers. Use Havoc on CD if the fight allows it and do not waste GCD's.
Please post logs though and we can pinpoint issue.
You can still post plain text. There's a stickied thread where you can ask for help analysing logs, I suggest posting there. Also check out the warlock workshops on Furty's Twitch Channel.