I just recently set a sound post and have done a little bit of homework so to speak on them. If it falls when you release the strings but sound good when they are tightened (tuned), there is nothing wrong. The post should make slight contact when setting it to hold it vertical from the back plane, and as little pressure on the 2, back and front, as possible. You don't want the post to damage the face or back from being to tightly forced into place. It is far better to have the pressure of the strings pressing down on the bridge then a sound post that is too long, even a 1/2 mm too long. It can alter the violins voice.

Sound posts are known to topple once all the strings are removed. It is recommended to never replace all 4 strings at once, Do them 1 at a time, unless you are ready and have the tools and skills to reset it (or a Luthier available)