Gummy wholewheat bread

Hey everyone, I am quite new here so please forgive me if this is in the wrong place or I am making some pretty novice mistakes, I need all the help I can get.

So a few times I have been making a 80% wholemeal sourdough bread and it has come out a bit 'gummy', but not sure why. 

So my Breadule (bread-schedule) that I followed for my last loaf is below:
Ingredients:
360g wholemeal granary bread flour
100g white bread flour
320g water

10g salt + 30g water + 1 tbl olive oil + 80g starter

40g seeds in 20g water
Schedule:

8 am: refresh starter with cold water and 50g rye flour
6 pm: add tbl rye flour + warm water to starter, mix flour with water (start to autolyse)
8pm: mix in starter, salt, water and oil. Knead in mixer for 3 mins.
8:30 pm: start S&F
9:30 pm: end S&F
10:30 pm: preshape
11pm: shape and in fridge
Day2
7pm: bake at 250 for 30 mins, 10 mins uncovered and left to cool in the oven.

Leave overnight to cool.

I can only guess I didn't leave it to bulk prove for long enough (I wanted to go to bed), is that possible? After the ~24 hour final prove in the fridge the dough had grown significantly and seemed quite 'billowy'. and the finished loaf looks good with what I'd class as ovenspring. In the past when i've made a similar recipe with similar results the bread has tasted very good, just needed toasting to remove the gumminess.

Any help or advice?