Assembly Warnings in Creo Parametric 2.0 (M120)

Frequently I am getting assembly warning with yellow indication (out of three Green, yellow and Red) in the Assembly status. After re-generation, it will turn to green. I will save the file and close the PTC Parametric window. Once I reopen the file, the Assembly status will again go back to yellow. What are the possibilities of error I am doing in Assembly. Can anyone in the Community help me out few tips to fix the issue?