Hi w1av.
I only have experience with POCSAG, so can't comment on the ACARS.
It usually means it isn't decoding the signal very well. In my experience this is most likely due to signal strength/quality or problems with the slicer.
Depending on your setup, you should be getting near 100% and full swing on the meter.
Can you describe your setup a bit more? scanner, discriminator tap or sound input, USB serial interface, PC OS, type etc?
Cheers
...Steve