Give them some time and they will flower for you. I've found that the MG I have in pots always bloom earlier than those in the ground.
I figure that the potted ones can only spread roots so far and then mother nature takes over to force flowers and in turn seeds to reproduce. The ones in the ground are developing a very strong root system and vine before they flower, which is what mother nature intended them to do.
It also can depend on the variety of MG you are growing.
Some japanese varieties bloom earlier than others, and your common varieties can bloom either earlier or later. Also our weather this year has been very strange with extreme temps.
It shouldn't be long before you are rewarded for your patience.

Ditto

You can also try a phosphorus based bloom booster...

Lot's of sun is a good thing. :) Arum

I also ditto what littleonefb said.

Also, over fertilzing can lead to alot more vigorous vine/ foliage growth. There is SOME truth to what many have heard about MG's "liking" poor soil. Its not so much they "LIKE" it, more that, its that nutriant poor soil will in turn produce less "healthy" plants, and the plants will force themselves into bloom and in turn seed, sooner so as to ensure its survival for the next year.

Personally, I love to fertilize because I LOVE HUGE HUGE vines with LOTS of lush foliage. Im willing to wait until july for blooms to get those awesome, huge vines with big dark green leaves.

~ NPG

I have lots of flower and seeds, and very few leaves! Just the opposite!!! Mine are in pots. Anyone want any seeds?

Here is a link that might be useful: Morning Glories Only