We are running multiple Dell Precision 690s with Quardo FX4600 video cards and we have been noticing sporadic tangent line display in our assemblies.
If you look at the attached screen shot, the blue gusset on the right shows it's tangent edges. The lighter tan part in the middle shows none of it's. And, the brownish orange cover at the bottom shows about 3 with 6 missing.
The only work around we have found is to open the detail part, edit any existing sketch, then close the part. Upon returning to the assembly, the tangent lines for that part will then show up.
We have tangent lines visible under view>display and under the tools>options>system options>display/selection.
Oh, we are running 2011 x64 SP3. We have noticed this problem on both our Windows 7 x64 and Windows XP x64 systems.