dunno said something about there being a rank buffer to get to blue
dunno's statement is correct but probably did not address your specific question. As per your rating page
your current casual rating is
Current rating: 2384.603 ± 156.0312
and the line between silver and blue is
Next rank bracket: 2298.347
so you have surpassed the line between silver and blue, but not by quite enough to rank up. Similarly, if you become blue and then lose a bunch of games, you would have to fall a ways below 2298 (or whatever the number then happens to be) to become silver again.
However, the rank bar on your player profile page takes this into account, so that does not fully explain why it said "Rank up on your next victory" and then you won and did not rank up. This probably has to do with WHR taking into account the results of other players, and games you are not in.
When you saw the message "Rank up on your next victory!" on your userpage the system believed you were at or over the required threshold to promote, but you can only promote when you win a game. By the time you won your next game, other results had come in which put you just below the threshold (either by decreasing your rating, or increasing the threshold).