Sunflower bud not blooming?

It may not be getting enough light.

If this is not a dwarf sunflower it needs to be in the ground or in at least a 5-gallon pot. It looks like yours is in too small a pot to continue growing.

Reasons a Sunflower Plant Will Not Flower

https://www.gardenguides.com/107324-reasons-sunflower-plant-not-flower.html

Here are some tips on how to get sunflowers to open: https://www.youtube.com/watch?v=Vys97ywWq6o

If you are having a lot of rain like we are it is not getting enough sunlight. I am having that problem too.

lack of sunlight is one problem and warm weather also

https://shesaidsunflower.com/sunflower-growth-timeline-life-cycle-8-stages/

Not sure where you live, but where I love sunflowers don't fully bloom until late summer. You may need to give it more time.

Hello. Good question. It looks like from this resource sunflowers can be harvested and 70 to 100 days so perhaps a little more time might be considered. Here’s a link for that review

https://hortnews.extension.iastate.edu/2018/07/sunflowers-–-symbols-summer

Regular watering may be required to get sunflowers established after germination. After establishment a mild fertilizer solution or a slow release fertilizer can be applied. Avoid high nitrogen fertilizers as they tend to promote vegetative growth and inhibit flowers.

Harvest

Sunflower seeds reach maturity 70-100 days after planting

Hi! I'm growing sunflowers on a sunny window sill. The buds take some time, but, are worth waiting for! Lots of sun, water when dry and it will bloom!

Last year we had a sunflower that took forever to bloom. It seems it just wasn't sunny/ hot enough for it. Once the temps warmed up sufficiently, it popped right open.

https://www.almanac.com/plant/sunflowers

it needs for sure more light

Try giving some plant food https://www.miraclegro.com/en-us/library/flowers-landscaping/how-grow-sunflowers