Why won't Blathers suggest the Museum Shop?

[Sanaki]

Alright so I have donated well over 20 items with at least one in each category. Why won't he suggest it?

 

[Reindeer]

You need to speak to him on 14 different days as well.

 

[Sanaki]

Oh.. welp. ._. Thanks

 

[Hypno KK]

Also, when you're done donating and talking to him, sometimes he'll only ask about it the next day. You can tell when he's going to bring it up because he'll have a thinking speech bubble over his head. Once I'd fullfilled all the requirements he didn't bring it up immediately, he only asked me the day after (and by that point I'd been talking to him for over 14 days).