Mine sometimes do this when they are either too wet or too dry.

~Chills

I have kept a careful eye on the passiflora and I am confident that it is not being over watered or under watered. It lives in a sunny garden in San Francisco with little wind. Does anyone else have any suggestions about how to solve the dropping bud problem? This plant has continued to grow and get stronger, and is producing many buds, but they still fall off before opening. I am very sad about this, and any help will be greatly appreciated. -- Karil

Hi Karil
Some Passiflora hybrids are particularly prone to this such as Passiflora x belotii. The best remedy is a high potassium feed. On the other hand you may have a species grown from seed which is never going to flower properly.
Myles

Here is a link that might be useful: Passiflora x belotii