before i get flamed to death here check this out. so i do have a clue of what im talking about
:
http://img194.imageshack.us/img194/553/6129dps.jpg
its no burst... boss is allrdy at 45% and without poping hero. got to 7.1 when hero was poped + fiend closer to the kill.
the only problem is see with single target dps is the relatively long "setup time".
we switch targets (if old target aint ticking with dots for some reason) we loose dps.
not like mages,boomkins,destros,ellies with relatively small set up time our dps mostly dots based starts ticking only like 15-20 sec into the fight.
thats around 7-8 sec of seting up all dots and stacks and another 7-8 sec to compensate for the first 7-8 seting up low dmg...
that means that any boss that resets (changes phases like xt heart, toc worms, razor p1, anubarak p1, etc) you have like 15 sec each "reset" before u truly start hitting hard, and if u can hit the boss for 45 sec before next reset that means your max dps will be ticking for only about 30 of his uptime.
now for your problem:
GEAR - if you were holy meaning most of your gear is spirit stacked forget about dpsing... you need spellpower and crit as your main stats.
i do not know what gear you have but im guessing its not dps oriented, so first get some dps gearing spellpower and crit max those as much as you can. as some one said 15% crit is very low.
and ofc glyphs...
Rotation - are you sure u keep all dots up? in the correct order? do you clip VT or DP? do you clip mindflay where you shouldnt ? or do clip it when you should?
most bosses arent single target do you multi dot and keep all offtargets doted (with bos)? do you pop fiend and other trinkets use on hero?
EDIT:
get improved shadow form as a shadow priest u channeling alot of your mindflay and each aoe hit steals from you one tick or so... on a 8 min boss fight thats alot of downtime.
and get a spd increase enchant for your boots if u dont have it allrdy. get out of bad stuff faster meaning more time for doing your job.