I have moonflower vines too. Mine also usually only open halfway but last night for the first time I saw one that opened pretty much completely open. I don't know what the difference is but usually my moonflowers open only about halfway as well. I will say that mine has never ever produced seed from its flowers, so you are lucky in that regard.

gardeniarose - Are the plants getting enough water...sometimes if the plant is somewhat dehydrated >the blooms may not open all the way and be floppy...
a cold spell could also produce flowers that only partially open,but I doubt that is the case in zone 10FLA...
if the roots are too wet >like soaking in water that could cause problems...
the only other possibilities are that it could be an unknown infection of some type(e.g. bloom wilt) or it could be that the particular strain (or individual plant) is carrying for blooms that don't fully open...

An excessive amouint of Nitrogen could be 'burning' the plant,but this is usually apparent in the leaves as well...

Try some phosphorus based bloom booster on it and see if that helps to strengthen the turgidity of the blooms...

TTY,...

Ron

Hi Ron,
I think if anything, maybe the vine has gotten too much water. Although it is in a good drainage spot, we've had lots of rain over the past month. The leaves seem to love it but I guess the blooms don't appreciate it. Summers are normally very rainy down here and last year's vines did great, but maybe we're getting more this year and I don't realize it. Thanks for the post!