In the past I worked for British Gas and had a desk near the Home Service staff in a big open office. They had an irate customer contact them because she had bought a new cooker just before Christmas and on Christmas day the turkey had been uncooked, despite following approved cooking methods.
Over the next few months there was a lot tooing and froing, thermostats were checked, test bakes of cakes and other items to standard recipes took place, everything worked perfectly. Everybody was bemused, then an Advisor asked to see the tin the turkey was cooked in and all was revealed. The turkey tin was large and when placed in the oven more or less touched the sides of the oven all round, so all the heat was confined to the area under the tin and very little got round the sides to heat the top. I was left wondering how often oversized tins might be the reason turkeys do not cook properly
So when cooking your turkey make sure the tin is small enough for oven heat to circulate freely around your oven!