I have 4 hydrangeas that are full of leaves and big but won't bloom. I never cut them back. The only time I cut anything was the first time. One did bloom and it was so gorgeous that I cut the flowers. That was 4 years ago and I haven't had a bloom since. All get full sun except for one that gets a little shade from a cherry tree.
You likely need to prune and your soil may also be lacking phosphorus, which is responsible for flowering. When pruning, make sure not to prune any new stems that grew this past year as these are for blooming next year. If you are not sure which these are, it may be best to wait until spring to prune stems. You can either use a phosphorus rich fertilizer or add some bone meal to the soil. These articles will help you:
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
I have numerous blooms on my hydrangeas, but they are not fully opening. The tiny buds around the edges are blossoming, but none of the center buds are opening. The bushes get plenty of shade during the daylight hours; perhaps they are getting too much to fully open?
5 hours of sunlight is generally a good amount for flowering.
You could try a boost of Phosphorous to encourage a flowers to open fully.
Bone Meal is a good way to do this.
You may want to do a soil test in the future to make soil corrections.
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm
We have a hydrangea that has bloomed for years and although it has flowers, it also has black spots on its leaves. What can we do?
Here is a link that will help you.
https://www.gardeningknowhow.com/plant-problems/disease/black-spot-fungus.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
The leaves of my 4-month old plant are appearing like they have something chewing on them. There are some new leaves and the older ones that have dark brown dryness and holes on them. Is there a product that is best for them to make them look healthy again?
I would use Neem Oil.
This works as an insecticide and fungicide.
Here is a link with more information.
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
I have a hydrangea that does not bloom. Why is that? What can I do to get it to bloom?
You may be lacking phosphorus. If there is a lack of phosphorus in the soil, plants will have little to no blooms. You can either use a phosphorus rich fertilizer or add some bone meal to the soil. These articles may help as well: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm
My Hydrangea bush has not bloomed for two years. It previously did have blooms. What does it need? What type of soil addition or fertilizer should I use to promote blooms?
Most likely you have to much Nitrogen in the soil.
This causes lots of greens and little to no flowering.
You can add Phosphorous by using Bone Meal.
Here are some links for you.
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm
I planted a hydrangea about two months ago and the leaves and new growth appear healthy but the old blooms have turned light pink with a lot of browning. Can I prune the rotting blooms? What is causing them to turn brown and how do I avoid this?
Hydrangeas can turn brown from heat stress, be sure to keep your shrubs deeply watered during hot weather.
You will want to deadhead the spent blooms, as discussed in the following article:
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm