Gap in upper two front teeth keeps opening up even with consistent retainer use and a wire on upper teeth

I went to an orthodontist 13 months ago to close a small gap in my two front teeth. I did two 10 week invisalign treatments, one for the top and one for the bottom, and he then placed a wire behind my upper teeth. However, the gap keeps opening up a bit during the day (wearing retainer at night). He says there is a fair amount of contact between my teeth when I bite, which is causing the gap to open. He recommended an additional invisalign treatment on the bottom to fix the bite. Is he right?