Help for tropical hibiscus wanted

Hello, I bought 4 tropical hibiscuses plants a year ago. I grow them as house plants because I live in Manchester. I take them out into the garden at the end of spring and return inside the house at the beginning of autumn.
They didn't look very nice at the beginning of the spring but now 3 of 4 are more or less fine. But the fourth one still looks rather miserable though with buds but they have not been opening for a long time. What would you advise me to do? Thank you in advance.

 

Hibiscus plants drop their buds for a variety of reasons and environmental factors can often play a role: to maintain proper watering, insert a finger into the soil and water when it feels almost dry or dry (this avoids under watering, over watering and inconsistent watering); double-flowered hibiscus begin to have flowering issues when daytime highs are consistently above 32C (if you observe that, as the weather begins to cool down in late September and October, the plants are now blooming again, you will have your reason); look for pests like aphids, thrips, hibiscus bud midge or gall midge larvae (you have to cut open several buds that have just fallen or are about to fall from the plants; the larvae of the gall midge are tiny and look like little maggots; use some Imidacloprid for some of these pests); or roots were disturbed.

Bring the pot inside when temps are near 10C or so. Place the pot in an unheated garage or a basement, with not too much sunlight as warmth and sunlight may make the hibiscus break their winter semi-dormancy too early. Supply only enough moisture so the soil doesn't dry completely but gradually increase the amount/frequency of water and sunlight in mid March/April so the plant breaks dormancy and begins to develop new growth (tropical hibiscus will not go totally dormant so it needs some water during winter when the potting mix feels almost dry). Expect to see some yellowing of leaves when brought to its winter location.

 

Thank you very much, Luis. I follow all these recommendations. The only thing which is different is that I treat my hibiscuses as houseplants.