Screen protector bubbles

I keep getting bubbles under the screen protector on my iphone 7plus. I didn't have that problem with the 6s. its already my third screen protector of a different company. the guys at mcshark shop told me it can not be the iphone it self, but there is no other option why it can be that bubbles keep rising and increasing in size. could it be that the screen is not straight ? in that case I have to send it to apple and they will change the screen or give me a new iphone ?

thank you very much in advance.