I am using ProTools Free on the Macintosh to record some stereo audio
from tapes and I had been using the 1 Band EQ II AudioSuite feature to
remove tape hiss. Up until recently, I would hear both the left and
right channels when I preview to check the effect of my filter.

Starting recently, I only hear the left channel during the preview.
Why might this be? What could have changed?

The equipment I am using is:

-A mac with a built-in sound input port
-A simpler mixer to regular the level
-A cassette tape player

The procedure I follow to make a recording is as follows:

1. Create a new session choosing 16 bit depth.
2. Choose New Track under File menu and add two AudioTracks.
3. Turn on the rec buttons for the two tracks and click the record
button (it begins flashing).
4. Play back the source and adjust the level on the mixer.
5. Play the source again, clicking the play button in ProTools Free
(along with the flashing record) to start the recording.
6. After recording, I group the two tracks for Edit and Mix, crop any
junk off the beginning and end, and fade in and fade out.
7. I play the track and if I hear objectionable tape hiss, I use a 1
Band EQ II filter to remove it (Peak filter centered at about 11.5
khz, Q often around 4.6). I use the preview button to check and adjust
the parameters of my filter as needed.

Thanks in advance for any tips.