Probably just a gear issue and lack of knowing the fights /strats me thinks. Gear plays a very huge role in heroics though and once you hit a certain Plateau its like robbing babies of their loot. I know my 3 frost dk and tank druid team is well over 5700 or higher gear scores yet the way I play them and the spec they use they only ever pull close to 2400 dps too, tho if I were to swap specs I could be pullin 5 k or higher. I like the spec I use for my dks tho and its easy to just loli gag Through instances clearing easily. Farm what you can, buy what mats you can and gear up at the auction house. Find items that can be crafted and get /farm the mats solo and give a crafter a yell . There are plenty of options