It's the weapon.

I see often people who struggle with Ret DPS, and oftentimes it comes specifically from one mistake - their weapon. From level 20, when you first get Seal of Command, to max level, everything revolves around the weapon in your hand. If you're running around with anything green, at all, stop immediately, run to the auction house, and buy yourself a weapon. If your blue is more than 5 levels below your requirement, you should seriously look into getting a new weapon.

Nothing has a more direct impact on you than your blade.

Second, this may be obvious to most, but its still important - there is no 1H weapon that should be used. Ever. Your DPS is simply not meant to use a 1H, so don't.

Finally, consider your weapon choices. Again look at stat primacy, but now you have a second thing to consider - the DPS and speed of your weapon. If you're a Blood Elf, the speed isn't particularly important, and you can use any 2H you like. If you're an Alliance Paladin, your only option is Seal of Command (with a few advocating Seal of Righteousness, but SoR only outdamages SoC in very specific clinical conditions. Not worth it overall). SoC is all about procs per minute (6.8 ppm to be exact), so the faster your weapon the less likely it is to proc, and more important, the less damage it will do.

Basically, SoC fires 6.8 times per minute, no matter what, and its damage is 70% of your normal weapon damage, period. If you use a faster weapon, which if otherwise equal to the slow weapon will have lower max damage, your 6.8 SoC procs will do less damage.

Long story short and getting out of the poorly described mathematical design, faster weapons literally equal less damage, and should be avoided at all costs.

So if you want to see results as a Ret Paladin, get a 3.5 or slower weapon, the slower the better, never let it fall far behind your level, and always keep it blue. If you practice this to the letter, you will hit far harder.