Best treatment for sour crop is a water only diet until the crop is empty then a dose of Avipro Avian. Repeated massaging with a little olive oil helps- using cod liver repeatedly causes an apparent vitamin overdose we discovered. Yoghurt isn't good, I think because lactose isn't digestible by chickens?
If the crop repeatedly won't empty on its own it is due to a digestive impaction further down the system. That could be for many reasons- eating indigestible objects, large objects, long grass or feathers, internal swelling or round worms partially blocking the system. We have had 3 this year during the moult, a time which seems to create digestive disorders, but only one was leaf stems and was treated repeatedly and eventually successfully. The other two were a Kidney infection and eating wood shavings -neither recovered.
Making sure chickens have plenty of grit minimises digestive problems as does regular worming.