Boxwood is supposed to be an 'ever-green', yet in the UK it's becoming increasingly noticeable over the past few years how they shed their leaves, especially on the top surface, over the winter months as shown in the two example pictures below. Yet starting in spring, new leaves grow to replace those lost: If boxwood is an 'everygreen' then why does it lose some of its leaves in the UK's winter months?
It usually means the Box is affected by one of the myriad problems it is prone to these days in the UK, ranging from infections such as Box Blight through to infestations such as Box caterpillar. Keeping them closely trimmed doesn't help with infections, but see here https://www.rhs.org.uk/problems/box-problems to try to identify the problem yours is having. Note that, with Box caterpillar, regrowth does occur once the caterpillars have left - you may need to wait till fresh growth arrives to determine precisely what the problem is.
