Not feeding sourdough starter enough?
My early sourdough loaves looked like that. Now they are less flat and have defined hikes through the crumb rather than a great interlinked cavern. 
The things that changed:


my starter became more mature and stable. It lives in the fridge and gets fed 2-3 times a week when I bake. I keep about 250g which gets 100g taken out and replaced for my standard loaf and 180 g on loaf-and-pizza days, plus another 60g at random times when my partner steals some. 
I’ve learned that in my kitchen a sourdough loaf takes a long old time to rise. Sometimes I bake it underproofed because that’s better than not having bread, but really mine needs 24 hours from start to finish. 
I gave up kneading. I read all the methods, watched all the videos... I slammed that dough down 300 times per loaf, kneaded for at least ten minutes, stretched and folded... and never got smooth elastic dough as advertised. Instead I got exhausted sticky dough that was a nightmare to handle and didn’t rise all that well. I’ve come to the conclusion that if dough has enough time it doesn’t require kneading. Now I mix the starter with water, mix in the flour with a rice paddle, mix a bit more about ten minutes later and leave alone for about 22 hours.  I might give it a poke if I’m bored, but that’s more for me than the bread. 
Shaping: I used to do a lot of folding/rolling up of dough before I put it in a proving basket for the last hour or two. Now I just kind of punch it down and gather it back up and dump it into a well-floured basket. 


I don’t have a current loaf to take a pic to prove my success (just back from Stratford, flying to Stornoway in the morning, had to actually do a discard feed for 1st time this year) but essentially my mantra would be ‘chill and give it time’.