Help with lievito madre/pasta madre not tripling

Hello - I've recently been having trouble with my pasta madre for making panettone. 

I initially converted it from a very active 100% hydration starter to a 40% stiff starter and it was great, easily tripling in volume in 4 hours but more recently it just hasn't been growing as much. It probably doubles but then stops. I do the refreshment by mixing 50g starter, 50g flour and 20g water and kneading until smooth and bouncy, then roll into a ball and cut a cross. The cross used to fully open out but now it doesn't - it expands a bit but doesn't fully rise up, even though I'm keeping it at 28°C

I've tried both vacuum sealing it overnight to simulate the "binding" that you sometimes see, or putting it in a cold water bath overnight. And also tried the bagnetto sweet water bath in the morning. However none of these seem to make a difference at all. 

Does anyone have any advice on how I can diagnose what's going wrong? There seems to be so much different advice out there that it's hard to tell what exactly is wrong. 

Probably worth saying I did manage to make a panettone with this madre recently, but the first dough took much longer than expected to rise and the final product I didn't get much oven spring (the top is almost completely flat).