Turkey frying oil temperature issues
If you are saying you cannot get the oil hot enough during pre-heating:


You may need a bigger burner than the one you are using. Most resources I've seen suggest over 100k BTU
There may be something physically wrong with your setup (i.e., the vessel should be closer to the flame)
The ambient temperature at the time of cooking was simply too cold for the burner to keep up (see problem 1).


If you are using a purpose-built commercial turkey fryer, I suggest consulting the book or contacting the company. If you built the rig yourself, I suggest more fire! (and lots of safety precautions)

Now... if you are talking about a temperature drop after you dunk the bird, realize that is totally normal.

Alton Brown suggests a preheat temperature of 250 degrees. He suggests raising the temp to 350 after the bird is in the bath but adds the following advice:


  "Closely monitor the oil temperature: it's will start to climb above 350 as moisture cooks out of the turkey; reduce the gas flow accordingly..." (Ref ep. 163 transcript in his book Good Eats 2, page 413)


You may need to use a smaller turkey to achieve the internal temperature you want without burning the outside. I'd look for something in the ballpark of 10 pounds. If you are cooking for a crowd, it's best to go with multiple birds instead of going bigger.

Be mindful of carry-over cooking:


  Even after it’s been removed from a heat source, food continues to cook. Its internal temperature can rise anywhere from 5 to 20 degrees. The larger the food, the more carryover or residual cooking you’ll have.
  http://www.cookingclarified.com/2011/01/carryover-cooking/