Geopak Drainage unable to calculate tin elevation, but DTM Height/Slope Analysis works fine. Thus .tin exists and in the correct place. Any ideas?

I'm trying to place nodes using GP Drainage. I am new user, so this may be a simple fix.  I'm trying to reference my .tin for elevations, but when I place my node it says "unable to compute TIN elevations". I know my .tin exists and is in the correct location because I can use the Height/Slope Analysis tool in the DTM tools and it finds the elevations just fine. Any ideas on what I may be doing wrong?