Signs are placed according to the Traffic Signs Manual (all 8 chapters). Often, where the signs should be placed is the most inconvenient, meaning a huge amount of costly accommodation works.
Unfortunately, there are occassions when the signs become overgrown by shrubbery. The current hot/rainy/hot again weather really doesn't help - have you noticed the same rapid growth of plants in your garden?
Regular checks (every 7/14/28 days depending on the classificaiton of the road) are carried out by experienced and well-trained personnel. I know. I used to be one of them
However, the defects are passed to an overloaded and very much underfunded maintenance department who then have to prioritise overgrown signs with 12" deep potholes and collapsing manholes.
Underfunding of the road infrastructure is one of the greatest problems facing this country - the money is never hypothecated (ring fenced) and is usually diverted by local authorities into education and social services.