I answered your questions with in the quotes ...
I had to add some kinda text hence the ,,,,,..lol

Tsk tsk tsk you are in danger of being excommunicated by breaching the quote rules..................

So vary your times
Ensure the dog will do longer than you need both out of sight and with distractions

You say you reinforce the dog with a hand signal? This is not what I would call reinforcement, I meant do you give him a reason to stay there by say feeding him in situ?

Go back and feed at random times, so you are giving the dog no reason to get up.

ALWAYS have a set routine so before releasing or commanding your dog. This makes it more predictable to the dog and increases his level of security (in the stays).

So I

Walk back to dog
I stand next to dog
I walk around the dog
I stroke the dog
I put the lead on the dog
I tug at the lead

I feed the dog during this so it is clear that what I want him to do is resist the temptation by deliberately tempting him (eg with lead tugs)

I then give a clear release word and then nothing exciting or rewarding for getting up.

there must be a clear distinction between what is going to be reinforced and what is not.

HTH

Have you been 'over-working' it, ie doing way too many repetitions in any one session?

Whatever the reason, go back to basics for two or three sessions and he'll soon get back into the swing of it

Do you exercise your dogs before training??

Its something i have thought about ..

Whether or not you exercise your dogs prior to training is, IMHO, immaterial as in my case you never know when you might have to do something.

Of course the dog should empty itself but a dog should do a down stay whether it has had 5 mins of exercise or 5 hours.

I tend to excersize before class, or my dogg is soooo zooomy she cant sit still.
She seems more focussed.
It depends on what your doing, agility flyball i probably wouldnt need to.
Obedience/KC certificate then I did.
I know people I train with dont as they like their dog to have the extra energy, but i have a HuskyX.
The zoomies tend to be toxic.
I did experiments to see what worked best.
When I didnt take her a run and she was doing her impression of the wall of death, thats when I figured that out