If you are cooking the meatloaf with a sauce or in another way, this may also affect the length of time it takes to cook. As part of the meal, if you are making other side dishes too such as roast potatoes for example that also need to cook in the oven, this may make it longer overall for both dishes to cook.
The shelf you use in the oven may also affect the cooking time. Try not to put the dish too close to the top of the oven as this may cause the top of the meatloaf to burn while the centre remains uncooked. In the same way, try not to put it at the bottom of the oven where it might take longer for it to cook. The best option is to place it on a middle shelf at 350.