7 Month old pup refuses to go poo sometimes.

As the title describes, my 7 month old German shepherd x husky will refuse to go poo on walks/hikes sometimes. 90% of the time he will poop right away but all of a sudden one day he will just decide to not go poo! He is fully potty trained BTW! He's never had an accident in the house thankfully but sometimes it seems like he's just holding in his poo for some reason? I cant figure out what I'm doing wrong, I've even started to add a little bit of pumpkin puree into his lunch and dinner. His stool looks healthy and there is no clear indications of any heath issues. Can someone please help me lol some days I wanna pull my hair out. I walked him this evening for almost 1 hour in a nice secluded field doing laps but no poo ): I keep him on a strict schedule too. I always walk him first thing in the morning, again around 11:30am, and again around 3:30 and one more time before bed which is usually around 7pm. I feed him on a very strict schedule too, so I have no idea what's going on. Some days he will just refuse to poo! Please help.