Milk Bottles Feeding With Diff Teats

I've let my LO to try on several bottles including brands like NUK, Avent & Dr Brown's. Now she's currently on Dr Brown's but I'm mixing bottles between the older version and the new version. I noticed that whenever she drinks with the older version, she will always play with the teats, pushing it around left and right with her tongue and too longer time to start drinking (even when she's hungry). For the newer version, she will also play but for only a few seconds and start to drink immediately after that. She's currently using 6 bottles in total (4 older version Dr Brown's and 2 newer version Dr Brown's). Her feeding is very irregular. Sometimes drink 40ml, sometimes drink 120ml. Am I confusing her with 2 diff teats? Should I just discard the 4 and replace them to the newer version one instead? I find it abit wasted cos 2 of the older version has only been used for like less than a month