Old towels are no longer absorbent! They are not that old, what happened?!
This is something that might happen over time as you keep washing your towels. If you are noticing your towels absorption has not been the same as when you first bought them. This might be the culprit "Fabric Softener" !!! Fabric softeners might add, overtime, a film of water resistant material that will limit the absorption capabilities of your towels. In some sever cases you will literally feel the waxy water-resistant film on the towels.
How to fix it? White Vinegar!
For a couple of washes, add a little bit of white vinegar to your rinse cycle. 1/4 cup should be sufficient. You will notice the difference right away, but do not make this the normal way of washing towels. You should only do this once or twice at the most and you will see the desired improvement right away.
To avoid this from occurring again, do not add fabric softener to your towels wash/rinse cycle. Or, if you like the short term effect of the fabric softener on the towels, but want to avoid its long term effect, repeat the white vinegar remedy once every few months or as you feel the towels are loosing their absorption power again.
P.S. This is also applicable to your cotton bathrobes.