No sound while recording

I don’t get sound while recording from an audiocassette-player. I can make recordings and play them back, no problem.
Working with an iMac 6.1, 10.6.8, Audacity 2.1.0.
With my MacBookPro 8.1, 10.10.5, I have no problem at all: excellent sound while recording!
I think I checked all I can think of. Please give me a good advice if available!

Enable Transport > Software Playthrough in Audacity.

If that does not work, get the legacy OS X 10.6 version of LineIn from Rogue Amoeba | Legacy Software (you will need to scroll down on that page).