Yep. The bars. Usually it's always the last event for my girls to be competition ready. But now I have MANY girls who are good on bars but lack the lower body strength so for them the floor is the one that holds them back.
In our experience, it was beam (for my DD). But for the majority of those I know who quit or completely stagnated, it was bars. I would assume vault wouldn't hold a gymnast back until ~level 8/9, because you can get away with FHS, even with a lower value, until then.
In compulsories it's always bars... In optionals it can really be anything depending on the gymnast. I personally think "not liking it anymore and wanting to quit" drives more higher level girls out than any physical limitation or skill though.
BARS!!!! My DD has ALL of her level 10 skills (and then some) but because of some fear issues on bars she will remain a level 9 this season (which I think is going to be the best place for her). But yes, I would definitely say bars.
I'm probably stating the obvious, but Bars seems to be its own special animal - most different from all the rest.
Natural, strong tumblers and twisters on floor often find success translating to vault, beam...
Flexible athletes fare well with leaps, walkovers and such on floor/beam...
Strength and dance skills like presses and spins translate to both floor and beam..
Bars is so different - swinging, timing, wrist shifting, catching... The whole sensation and mastery of orienting and flipping around an object (the bars) in 360 instead of basically in a straight line across a horizontal surface...(floor, beam, largely vault). Sure there are some skills like presses than can aid in a cast handstand, but overall, just seems to be the event that stands out as requiring, on the whole, a completely different set of talents and skills. So it makes sense that it is most common to have Bars be the outlier that is either your strength or your weakness compared to the other three.
I'd say men's pommel horse holds this distinction in MAG - the event where being skilled on other events doesn't really translate (enough) - whole other set of techniques to swinging pommel.
Hmmm....other than BARS at every level, I would say
Level3 - round off backhand spring on floor
Level 4 - kip
Level 5/6 - backwalkover or backhandspring on high beam
Level 7 - Giants and vault
Level 8,9,10 - really individualized road blocks from this point on. New skill/body changes/fears
For my dd it is the scary beam! She is always held back because of beam fears especially going backwards -walkovers then bhs and now the series. She gets skills then gets scared again. She could be a strong level 8 on all the other events but is competing 7 because of beam...