Hi maybe the teat is too big rather than too small? My gal is 10.5mo and she is still using S teat for wide neck bottle. My niece who is 5yo is using M teat. Does you baby find difficulty in sucking? Did she always have a hard time finishing or only recently? My nephew who is 8mo sometimes has problem finishing up milk too but he tend to finish most of it after a few tries. Some babies are like this. They don't finish at one go. So you have to try again later or distract with music. Sometimes my nephew prefer to sit in his rocker to drink milk rather than have people carry in their arms.
Personally I prefer pigeon teat. The slower flow prevents my girl from getting choked. Just that at this age, they may be exploring their surrounding and will take longer time to finish their milk. It could also be due to the solid that you may be introducing. When they r full, they will have a reduced appetite. So will drink even slower than usual feeds.
For piegon, they have a teat that is the Y teat for thicker liquid. You can try that.
Pigeon teat is one of the slowest. My 8 months old is still using S teat :)
It could also be teething