I've just stumbled on the same problem and found this: Symfony2 FlashBag stopped working after upgrade to 2.4?
If the other subject doesn't answer your question you might want to try and dump your flashbag to see it's structure. Do do so, try adding this in your twig template:
{% set array = app.session.flashbag.all %}
{% dump(array) %}
You might be surprised by what comes out, at least I've been:
array(1) { ["test"]=> array(1) { [0]=> string(28) "Ceci est un test de flashbag" } }
Which means that you do have messages in your flashbag but you can't get the content the right way as it's in a second array. My solution:
{% for tag, line in array %}
{% for underline in line %}
<div class="{{tag}}">{{ underline }}</div>
{% endfor %}
{% endfor %}
Hope this helps