Often, when my flock was young, they were very wary of every treat I tried to feed them so my advice is, just keep at it.  As suggested, chop up hard veggies like the Brocc, Cauliflower, carrots, etc.  I even chop up the lettuce and spinach that I feed mine.  Bite size pieces mean less risk of things getting stuck in the crop longer than it should.  
I also make a treat mash for my flock made with oatmeal, groats, a little ground flax, chia seeds, red wheat and a 10 grain cereal.  I mix it with canned pumpkin and some molasses.  It keeps in the fridge for a good week.  They love it and it's cheap to make because I buy everything bulk.
When they get a bit older, offer them meat.  I give mine rinsed canned tuna occasionally and they go wild for it.  I also feed meal worms.  
Know that eventually, they will eat the treats you offer and will most likely, look forward to treat time.   They're just slow to try new things when they're very young but eventually, their curiosity wins out and they start tasting stuff and then they're hooked!
Usually, it takes one brave chicken to try it and then they all want it!