Creatures have varying degrees of needs for each of the care actions you provide. The amount of DNA you receive depends on how much the creature is craving what you provide. 

For example, a hungry creature will provide a lot of DNA the first time you feed it but will provide less DNA the second time in a row that you feed it. On the third and subsequent attempts, you will receive a minimal amount of DNA.

The needs of creatures fully reset three hours after the first care you provided. This means that if you wait for three hours, the creature will give the highest amount of DNA it can for that specific type of care action.

Note that you can always preview the amount of DNA you will receive by pressing and holding on each care button.