Considering you give no reference, example or even offer explanation I'm going to say yes, your mixing is thin...

Seriously, got clue?

Thin is a frequency term, so you can fix it with an equalizer generally. However, If you use good microphone selection and placement, you shouldn't have to use as much EQ (or any). To fix thin sound, choosing instruments that fill out the bottom end will help.

Do you have monitors? If not that could be an issue. It's very hard to mix the low end without them. Before I had monitors my mixes were constantly either muddy or thin.

Listening to your music it seems that you use a lot of samples. So instead of microphone selection and placement, sample selection would make more scene.

These are very broad suggestions. Posting a link to the song that is getting the "thin" comment may help narrow the problem. (as Greener suggested)

From [[url=http://[/URL]="http://www.recordin…"]The Recording Institute Of Detroit[/]="http://www.recordin…"]The Recording Institute Of Detroit[/]

Thin Sound - A quality of sound of not having all frequencies present especially a deficiency in low frequencies.

Fat - Having more than a normal amount of signal strength at low frequencies or having more sound than normal (by use of compression or delay).

Your listening environment could also be a factor.

I guess it's time to get monitors. I've been doing it all on headphones but really nice ones, though.

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is my post in mp3 clinic