For several years, at rosybee, we have been trying to support solitary bees by providing nesting tubes of various kinds in a 'bee hotel'. Each year a few of the tubes are used but really nothing to get excited about.
Then this year I have invested a few new resources bought on-line; mostly bamboo cane tubes with nicely drilled holes of varied sizes. To my surprise the first new set of tubes was almost completely filled by the end of May. I got really keen and bought some more .........which have been completely ignored. I think I have now missed their 'nesting window' but at least I have managed to attract more than usual.