Shimano left shifter won't shift up to big ring

in Road general
I bought some Shimano Ultegra (ST-R8020) double hydraulic shifters in May last year. After a couple of months, I noticed an intermittent problem, when I wanted to move up to the big ring the lever wouldn't move, it felt like I'd break it if I pushed too hard. After riding a few hundred metres, it would suddenly allow me to shift up. This carried on with increased frequency until I can no longer move into the big ring.
I've changed the inner cables twice and the outers once, moved the front mech by hand to check it moves freely, checked the chain doesn't foul the chainrings, flushed the shifters out with a silicone lubricant but nothing makes a difference. If I disconnect the cable and hold the cable while moving the shifter, it appears to work so it's only when it's under load that it won't shift.
I've setup loads of front mechs on all my bikes and never had an issue like this. Has anyone had a similar problem?
I've changed the inner cables twice and the outers once, moved the front mech by hand to check it moves freely, checked the chain doesn't foul the chainrings, flushed the shifters out with a silicone lubricant but nothing makes a difference. If I disconnect the cable and hold the cable while moving the shifter, it appears to work so it's only when it's under load that it won't shift.
I've setup loads of front mechs on all my bikes and never had an issue like this. Has anyone had a similar problem?
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The claim may take a couple of weeks and there have been supply issues with the R8020s, both left and right. You don't say where you bought them but your local LBS may run the claim and the claim should include the cost of fitting.
I bought them new from www.bike-discount.de and I emailed them to ask if they could help. They sent me a prepaid label and said the claim with Shimano will take at least 5 weeks. That's a long time to stay in the big ring π
Might have to buy a cheaper (105) one from somewhere and fit that π
Sounds like ypurs has gone unfortunately, so warranty replacement.
One minute it was working the next it wasn't.
My new one still does it now intermittently.