My bite plate doesn't fit my teeth anymore but why won't my orthodontist change it?

It is difficult to know what you are referring to by "bite plate". My assumption is that you are talking about a night guard (a device worn, generally at night, to protect your teeth from the effects of clinching and grinding). If you had the night guard fitted before orthodontic treatment began then as teeth are moved during orthodontic treatment the night guard will no longer fit. In fact, if you are trying to wear a night guard while doing orthodontics, the night guard will work against the orthodontics; i.e. the night guard will try to move the teeth back to their former location while the orthodontics attempts to move the teeth into their new, more desirable, location. Discuss with your orthodontist your concerns and come to a mutual agreement as to what you should be doing about wearing your "bite plate".Good luck!