Try to keep all head/cornet/embouchure movement to a minimum, as physical movement is generally slow and can restrict the speed at which you 'flex'. Some( a lot ) players press harder for the higher notes, again the 'pressing' movement is involved in the 'flex' and can reduce the flexing speed and success, and also affect your stamina. It sounds to me, having not 'seen' you perform this nightmare flex, that there might be quite a bit of pressure change, from top to bottom. After pressuring your teeth down your throat, strangling your embouchure, you are then 'falling' to the low note. The change in embouchure exertion to no pressure, means that your lips + muscles are trying quickly regain their relaxed fatness. This doesn't happen quickly after being squashed to kingdom come!! I could be utterly wrong of course.
How to fix it? Well there's no quick fix, but you can certainly reduce the pressure quickly and maybe have that passage sorted out by the contest.
Two main things happen during a 'flex'. A change in muscular 'tension' of the embouchure and change air flow, more/less support from the diaphragm. All the work done during a flex is done by your embochure and air/diaphragm. If you do anything else, chances are you'll be hindering either of these two mechanisms. Work on changing the embouchure tension WITHOUT changing pressure, i.e. use the muscles of your face and not your arms!!( There's more on this in another thread, somewhere) Practice diaphragmatic accents and starting notes without the tongue, by 'hooo'ing. Then go thru the vizutti 'mare' flexibilities, simple ones first. It is important to have good control of these two 'spasms', you need to control when to 'spasm' and how much to 'spasm'. The duration of the flex should be as quick as possible, even when playing crotchets. When you play semi-quaver flexs, you do the same thing just more often, not any quicker. All this I'm afraid will take practice!!( Groan). Remember, don't just practice the bars you need to get right, but practice the technique behind the bars, loud, soft, high, low, etc..
I hope some of this diatribe will be of help, and good luck with that bit at the contest!!
