There are several factors that can cause a well made flute to play flatter than A=440 hz. One factor is room temperature.
The A=440 hz tuning standard assumes a room temperature around 20 degrees C. If you have a flute that was designed to play A=440 hz at a room temperature of 20 degrees C, but you play it cold in a room that is only 15 degrees C, say, it will play flat (around 15 cents flat, or something close to A=435 hz). There is nothing wrong with such a flute. Its just behaving naturally in a cold room. If you have a tuning slide you can try to compensate for this.
Similarly, but to a lesser extent, humidity also affects pitch, because it affects the speed at which sound travels inside your flute. A low relative humidity (lower than the humidity at which the flute was made and tuned) will make your flute play flatter. Low humidity is very common in winter, especially in cold areas with centrally heated houses. During cold winters in certain areas of North America, for example, relative humidity inside homes can become extremely low, and be problematic for wooden instruments. If I remember correctly, you are in Montreal. Is that correct. That would be a very cold environment with very low indoor humidity in winter.
Very low relative humidity will affect the wood that the flute is made from, causing it to dry out, shrink, and potentially crack. If you play such a flute for a long period of time, the moisture in your breath will rehydrate the flute, but probably too quickly, and this too can be a problem, and can cause damage. Lose tenons might be a symptom of a flute that has dried out too much. Swollen tenons that jam in the sockets might be a symptom of a flute that has gathered too much moisture, perhaps as a result of bing played for a long time in a cold room (causing a lot of condensation inside the flute).
The solution to the drying out problem is to keep the flute in a humidified container (to prevent it from drying out too much). Ideally, you’d store your flute at a similar humidity level to the one in the environment where it was made. The solution to to rapid intake of moisture is to warm the flute up slowly before playing, and to oil the bore regularly, to prevent the flute from taking on moisture too quickly when you play it.
Other factors that can make the flute play flat are leaks, perhaps due to drying out and shrinkage of tenons, or cracks, both of which can result from letting the flute get too dry. Also, the way you blow the flute can make it play flatter or sharper than it did when the maker tuned it.