Heating

Some Deep Fryers heat up faster than others, please see details below for pre-heating times:
Non-Immersion Element (No element in the Oil): 25 to 30 Minutes
Immersion Element (Element in the Oil): 7 to 10 Minutes

Are you by any chance overloading the fryer? try smaller batches.
Preheat the oven to 180 to 200 F.

Heat the oil in the fryer to 280 or 290 F. In small batches appropriate to the size of the fryer (fries for a serving of 4 people may need to be divided into 2 or 3 groups), fry each batch for about 8 minutes at this temp. Remove to a holding bowl. They will look limp, nasty and oily. (This uses 1/4" square fries from baking pototoes cut with a standard mandoline.)

When all the fries have been precooked, increase the oil temp to 375 F. Add the fries back to the oil in the same sized batches as before. It will take 3 to 5 minutes for them to crisp and brown. Drain, salt, and move to a cloth-lined bowl and hold in the 180 F. oven. Let the oil temp recover to 375 F. and cook the next batch in the same manner. Repeat, holding interim fries in the 180 oven, until all the fries are done.

When all batches are done, toss to distribute salt and the new fries with old, and serve immediately. Properly done, all commercial french fries pale in comparison to these.

However, I cannot guarantee that this method works with frozen commercial products. I always use fresh baking pototoes. That said, the frozen product should work to its optimal advantage when done in small batches with the first batches held in the warming oven as the other batches are done.

If it still doesn't reach 300 F then by all means take it back to the shop and ask for a replacement. If you're out of warranty get an estimate for a repair.

Good luck and bon apetite ;]