Entire flock impact crop?

[Rookie Raiser]

Hi! I have 23 (3 month) old chicks and all but 5 seem to have impacted crop as they have the hard lumps on their chests, but the 5 noticeably do not have any kind of lump. We unfortunately had one pass away this past weekend from it.
11 have been in the coop for 2 weeks, while the other 12 have only been out in the coop (out of the brooder tub) for a week and we just noticed tonight that a lot of them have it.

I have separated out the 5 that don’t seem to have it, and I took the feed out of the coop of the ones that do and put in some grit I had.

Is there anything else I can do? I will try the oil and massaging tomorrow but unfortunately cannot tonight.
I do not have a photo at the moment, I’ll get some tomorrow as well

I should also note that I have had chickens for 10 years now and I have never seen this in any of my chickens in the past

Any help is much appreciated!

 

[Wyorp Rock]

Re-check their crops first thing in the morning before they eat/drink to see if they have emptied.

You're providing grit for your chicks right?