If you have problems now, and did not have problems earlier
and if all the eggs are from the same flock each time (your own)
Then think about anything that CHANGED between the ones that were fine, and the ones that were not.
Did you change heat source? Bedding? Different food for the parent flock?
Is the brooder in a different place? Is the unheated part of the brooder the same temperature it was before? (Seasonal changes, heating, and air conditioning might change the temperature where the brooder is.)
If you had been using a heat lamp, did you change the bulb between batches of chicks?
Check for mold in the bedding, the chick food, the parent food.
Check for things like cleaning chemicals or air fresheners, car exhaust, fertilizer or pesticides that may have drifted, etc. (Depending on whether the brooder is in your house, garage, shed, or outdoors, you might be able to rule out some of those quite easily!)