located outside with direct sun.
This is a complicated subject with many causes. This article will help you to cross issues off the list, one by one:
I have some hibiscus plants, the ones they sell at places such as Lowe's or Home Depot. The blooms were fine until I planted them (a total of 4 plants) directly in my garden. The blooms grow big, but fail to open. What mineral if any are they missing?
This is normal transplant shock. You should not do anything to the soil until they recover. This can take up to a few weeks. Keep in mind that these are heavy feeders, and will require fertilization often. This article will help you to care for hibiscus: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm
I upload a picture of original
I will help to have a picture of what they look like, now, also. Something like this will be either a very slight phosphorus deficiency, pH imbalance, or a drastic change in lighting from what the plant was used to in the past.
Here are some articles that will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm
Before they bloom, buds look like cut off with scissors. Don't see bugs but sprayed anyway. What could make them fall off before blooming? Thanks!!
There are many reasons for hibiscus bud drop: insects, nutritional issues to name a couple. Here is an article that specifically addresses bud drop on hibiscus shrubs:
Healthy leaves, no visible insects, sUnny location, daily watering.
These plants are heavy feeders, and they will require near constant fertilization. Bloom drop will happen with improper feeding. This article will help:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-fertilizing.htm
I want to start a Hibiscus plant from seed...How do I do that? Or can I take a cutting an root it in water?
Yes you can propagate by cuttings or seed. Planting the cuttings in a potting mix is recommended but you can try it in water, too.
Here are the instructions:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-propagation.htm
What is the problem when the male anthers don't have any pollen on them? Water? Fertilizer? Temperature? It's frustrating when I got to hand pollinate and there is no pollen!
Actually, pollinating will, immediately, send the flower to die. After pollination, the seeds will, then, be the most important thing for the plant to focus on, and the flower will start to decay. Holding off on pollination can prolong the flower, sometimes.
Unfortunately, too much, or too little water can both cause this as well as too much nitrogen in the soil.
These two articles will help you to address the problem.