I find in IoS when sharing a note to Evernote if I try to add a tag by searching all my tags then tags that I know I already have saved in Evernote and are searchable within Evernote app do not show in the list and as others piling out it is impossible to add a new tag at this stage (why ever not) I suppose they prefer you to be using the Evernote App but the share facility is so much easier.