Have you requested the procurement file? The procurement file typically contains the solicitation, each of the vendor's proposals, an evaluation memo, and the final contract, including what terms and conditions were negotiated.
Did you miss a requirement? After reviewing the file and evaluation memo, you may discover that you were deemed noncompliant because you missed a requirement. The good news is that this is an easy mistake to correct in the future.
Did you fail to capture their attention? Did your proposal generate interest and was easy to follow? After reviewing the winning proposal, you may realize that your proposal did not get to the point, or did not adequately convey that you understood the project.
Where did your competitor excel? With the procurement file in hand, you can now dissect your competitor's value proposition and pricing. All of this information is competitive intelligence that you can use to your advantage. But, remember this is a two-way street.
Have you requested a debriefing? The procurement officer may not give you a lot of detail, but you will learn valuable information about how you stacked up against the rest. Plus, you reinforce that you took this project seriously.