@Rasser,
I'm a little late to the party here, but in case you get a moment to check this, I'll give you a quick overview of my personal insights and research from playing a warlock.
Requirements
Rotational competency - means you need to be able to perform your rotation in a variety of situations without focusing the entirety of your attention on it.
Bossmods Addon - you need to be tracking environmental occurences so you can move and use CDs accordingly.
Personal/Raid CD tracker - this is less important but still makes a big difference; you need to know when your major procs are up, and be watching things like lust, the ring, etc. I use Weakauras.
If you have those things, I recommend then working your way down the following list and investigating each step. When I say 'check', what I effectively mean is - find someone in similar circumstances (use warcraftlogs) and compare yourself to them.
Example Link:
https://www.warcraftlogs.com/rankings/8 ... bracket=15
Check
Check rotation - use websites (icy-veins is a fine basic resource), ensure you haven't made a basic assumption mistake, and look for tips on optimization, even if you think you've nailed it.
Check talent/glyph loadout - its fairly homogenous this expac but in certain places, having a suboptimal talent array can impact your numbers.
Check ilvl - dont bother making comparisons to people in other ilvl brackets, scaling with ilvl is enormous this expac. I recommend using warcraftlogs (grievance uploads all raids) and comparing your numbers to those of someone with a similar ilvl.
Check tier - tier slaps on a ludicrous amount of DPS, we're talking minimum 10%, much more in certain cases.
Check ring - another 5 - 8% under normal raid conditions, or absurd numbers on bosses that are farmed by groups with high average ilvls.
Check trinkets - DSI, Class trink and the Orb of Voidsight will all situationally alter DPS by significant amounts (I'd ballpark 5%, but that's completely out of my ass. There are stats around on the web if you care to investigate).
Those are the basics. If it all looks kosher, the next bit is a little more complicated and requires more time.
Check ability uptimes and distribution - find someone of your spec and ilvl on a fight of similar duration, who has good numbers, and do a side by side comparison of ability distributions and uptimes. If yours are different, investigate why.
Check ability timings - this is HUGE. Here's an example...
Me:
https://www.warcraftlogs.com/reports/V2 ... s&source=3
97th Percentile Parse:
https://www.warcraftlogs.com/reports/pP ... s&source=6
Those are the buff uptimes for myself on my fun-run with Grievance last week, and a warlock who parsed very highly. I did fine... but only 60th percentile for my 726 ilvl on a 4:11 fight duration. He parsed 97th (for bracket) on a 4:30 fight. Gorefiend DPS-whoring is about maximizing feast DPS. If you look at my buffs, they're garbage. Late potion and doomguard use that missed my first DS and missed the ring. Abysmal. Look at the spike damage - I went to 400k dps when the ring popped. The 97th percentiler popped all his abilities on the mark and hit 750k dps when the ring popped. Massive difference. (@RLs: I do apologize, that's embarrassing and I was doing it all run. Very rusty, haven't raided properly on that toon in months

)
It's by no means infallible, but by investigating these things you should be able to zero in on any significant problem areas you're having.
I'm happy to chat in more detail if you'd like. Contact me via message here, or find me in game.
-Disapproving