Just bought my beatsX about a month ago. Had this same issue. Apparently a persons ear canals can change over time. I used to be able to use medium sized tips but now the medium is too big for my left ear and the small is too small for my right ear. Look on amazon for complyfoam ear tips. They have a set made specifically for beats earbuds and they are fantastic. They work like ear plugs. Just put them on, roll them to a small tip and put them in. The heat from your ears will form them to fit and they won’t fall out. They also do a better job at noise isolation. $20 for 3 pairs. One small, one medium, one large.
They work flawlessly. Block out unwanted noise and hold my beatsX in my ears since the tips that came with them can’t seem to do the job.
I just ordered a set of comply ones from a review I saw on r/Apple and how well they worked with the pros
Comply Sport Pro Premium Memory... https://www.amazon.com/dp/B06XJLYH6F?ref=ppx_pop_mob_ap_share
I wasn't sure which size was going to be best for me so I ordered a set that had S/M/L, the large ended up being the winner. I really like the Sport Comply Tips https://www.amazon.com/gp/product/B06XJLYH6F/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1