While making .stp or .igs files from Creo .asm file how to avoid data loss problems.

My understanding is that IGES and STEP formats contain surface data only.  When you import one of these files, Creo takes the surfaces and converts them into a solid if it can find a solid volume.

However, if there are any errors in the translation process, the surface model may have small gaps, overlapping surfaces or other problem which prevent Creo detecting a closed volume, and so it doesn't automatically solidify.  If you modify the surfaces, either by manually creating surfaces and merging, or using Import Data Doctor, you can often then add a Solidify feature to end up with a solid component.

In some cases, simply altering the accuracy of the imported model may be enough to allow it to solidify.

In general I believe that STEP is less prone to errors than IGES, but I've often seen problems even with STEP when exporting / importing between two different CAD systems (Creo and Catia don't seem to play well together).  I'm surprised that you're getting problems re-importing a Creo export, though.

What is your default accuracy setting?