At the risk of assessing a problem without having the opportunity to see the skills being performed personally here are my thoughts to why they are letting go too early:
Often gymnasts learn a Moy by swinging back arched from support position above the bars and holding that arch as they extend their shoulders open to initiate the bail to long hang swing. This arching creates forward motion through the bottom as the gymnast tends to kick forward as opposed to up. Think of it in terms of high bar dismounts, if the arch comes too soon through the bottom then the subsequent kick will send the gymnast too far forward from the bar sacrificing height during a dismount. The Moy is similar to this concept.
To combat this I recommend two drills:
1. Start with the feet on the bars, in a more open shouldered press up shape, then perform the bail to long swing Moy. By doing this you can pause the action of bailing and emphasise the position the gymnast should be in before they swing through to hanging position (a nice hollow shape with shoulders extended to cover the ears)
2. Get two blocks and place them side by side either at the end of the parallel bars or at the front swing side of a high bar. This setup will simulate a tunnel bar station. The gymnast can then swing and perform the Moy, either from the Parallel Bars or a High Bar and land on his upper arms on a soft mats as opposed to the hard wooden parallel bars. The drill can be made more difficult/effective by getting a higher stack of blocks than the height of the bars to emphasise a correctly performed tap, and rising from the bars to a higher upper arm position then will be necessary on the actual Parallel Bars.
I use both these drills independently as well as together for beginning progressions to not only learn Moy's to upper arms, but Moy's to support as well. Furthermore, the blocks can be used to simulate Tippelt's when the time...Hope this helps.
Nice drill, Blanton. I asked a similar question on DrillsandSkills a few months ago as our season was finishing up and unfortunately another coach had showed them Moy's besides them seeing the L6's do them.
I didn't know what to have them work besides spotting it carefully. We had already worked bails from a pushup position to a cheese mat in between the bars. I picked up this drill a long time ago from the USAG BSAP L3 skills. I was just trying to have them focus on not arching so much. We did a bunch of casting to bails in straps trying to get that nice hollow instead of just an arch that drops.
Ever work swings to a straddled front uprise? I was having them straddle their legs and with my spot figure out how their body and arms have to work to get to the support ( upper arm or support ).