Baking – bake 1 round of puff pastry on top of another

If you've baked your puff pastry in the past at 375 then it's probably not a temperature problem, however I usually bake it between 400-425 degrees (400 convection or 425 in standard oven). Lower temperatures aren't a problem for the baking aspect as it will still cook through. The problem lies in the fact that the heat won't be significant enough to generate the steam needed for expansion.

Egg wash could be a culprit as it should only be used on the top and not allowed to run down the sides where it will seal the edge together and keep it from rising unevenly.

The Pepperidge Farm site offers some tips for baking related to the rise:

• Always preheat your oven for a minimum of 15-20 minutes before baking, because Puff Pastry depends on even heat to rise and puff. Place pastries 1 inch apart.

• If you want a flaky thin and crispy pastry that's not very puffy, prick the unbaked Puff Pastry all over with a fork, which lets steam escape while baking.

• To create a tart with an extra puffy crust: take a knife and score two lines around the edge, then prick the area inside this border with a fork.

I would begin by verifying that these tips were observed. Also, I would verify that your oven bakes consistently (i.e. no hot/cold spots). Preheating can be very helpful in resolving this. Also, you can put in a pizza stone or other heat regulating product, and monitor the temperature with a probe thermometer in water in different areas of the oven.