Blue tit nest building

Hi, I have 3 camera nestboxes spread out in my large garden. Nest box 1 was used for roosting during winter , then in late Feb nest box 3 was regularly prospected. The pair started bringing materialBlushin on March 14th and the female started sleeping in the nest about 10 days ago aswell. The nest is visited multiple times a day by both blue tits, and as of today they are still bringing in feathersBlushI was told it would take anything up to 2 weeks, however it is nearing a month . Is this normal? I'm starting to think she will never lay an egg?! An advice greatly appreciated Blush