Siamese fighters

Sceilg,
Firstly this is only one person's experience with them and may be at odds with others' but for what it's worth a bubble nest can be fairly small, but if there are plants at/on the surface to help the male 'bind it together' it will not 'collapse' for want of a better word. So possibly this could be either his first attempt or just without any 'binding' material. You could try a floating item, even a small 'square' of Polystyrene like a piece of a ceiling tile or an offcut of a sheet of Bubble-Wrap, but I always found the plants to be the most successful - some Riccia or even Duckweed would be OK but any floating plant will do.
Female Bettas invariably show the horizontal lines most vividly when ready to breed, another sign is often the 'EggSpot' at the vent, I have to admit to not knowing the purpose of this but evidence of one can be a significant pointer that the Female is ready.
If your water is very hard this might be a factor, but as the male constantly adds to the bubbles you generally get a good nest eventually.
I have found Bettas are not very tolerant of high pH in their water, this can be significant, what's your water like where you are?
Is the female welcoming his advances? - You will need to remove her after egg-laying is complete as the male will become very aggressive toward her, on around the third day you will also need to take out the male as I have found the paternal instinct tends to diminish arouind that time, the fry then becoming a tasty treat!!!

You are rushing a little ahead of yourself asking about fry food, but the fry, when they become free-swimming are relatively tiny, I would definitely recommend Infusoria as a first food for Betta fry, moving on to Microworms/Brineshrimps after a few days.
And - even further along the line - yes, they DO need to be separated once the bickering starts and yes, they do need heated water, they aren't the most tolerant of fish to very cool water - they don't grow on very well in it either, here's the time when you need either loads of suitable accommodation or to be VERY positive at culling, doing away with the unwanted males - sounds horrible, but I'm afraid it NEEDS to be done.
You will probably already have read that the tank containing the fry will need to be tightly covered to prevent draughts at the water surface, the fry take their oxygen from the water in their early times but when they start to take in air from the water surface it really needs to be at the same temperature as the water is, I've lost fry at this stage - they truly are pretty delicate little lads for a while.

Keeps us informed how things progress.

John

I have read that the female gets vertical stripes when ready?

Perhaps mine didn't read the same articles? ;o)

The male never blows bubbles into his little nest while I am looking at him: checked the tank-no nest. Checked back an hour later-cluster of bubbles, but still doesnt show interest in the nest while im around.

He's probably bashful...seriously though if it's a 'first time' male he'll be a little confused...Have you had the pair long - and do you have a high pH? - Indian Almond leaves are highly recommended by many, I haven't tried them yet with Bettas but find them admirable for lowering pH with my attempts to hatch Killifish eggs.

John

I used to find it best to be slightly on the acid side of neutral but if yours are tank bred your pH should be OK.

I suggest you leave them together perhaps another day then take away the female for a couple of days...if possible give her either live food or frozen if not...then try them together again - absence, as they say, makes the heart grow fonder...

John