Offering it and actually asking them to do it are two different things. You must be direct. Next time, I'd reiterate what you would be offering (don't just say "a longer lesson", but include what that lesson will include that is currently not available in 30 minutes). Then I'd ask something like, "I'd like to start this program for Susie this fall. The cost for this program is $$$. Is that OK with you?"

Each summer when I send out my fall enrollment form, I have parents mark their tuition plan. You could do something similar and have the 30 minute and 45 minute programs already "pre-selected" for each student on the form. If they don't want to do it, they'll let you know.

I teach only 45 minute lessons so I don't have this problem, but this is how I might approach it.

This past year I had three students who I knew would really benefit from 45 minute lessons, yet they were doing 30 min. They were at CM level 2 and Guild in the Spring. Throughout the year I would comment periodically to the parents (as they sit in on their child's lesson each week) that we were really 'crunched for time' with such a short lesson, or that 'next year CM lev 3 will need much more preparation with theory and technique', etc... letting the parents know that 30 minutes just wasn't cutting it. So the parents kind of knew all year long that NEXT year we'd be going to 45 minutes. I'd prepared them way ahead as to the cost and by the time registration for the Fall came around they didn't blink and signed up.

Anneliese, when I switched over to all 45 minute lessons, I didn't give the parents a choice. I already had about half my studio in 45 minute lessons, but I knew the others would not do it unless I made them. For a year, every new student was not given the option. I only quoted the 45 minute tuition. I took in a few more students than I really wanted, so that if some parents got upset and quit, I would still have a full studio. In the spring I sent out a letter outlining my new policy and the very good reasons behind it. I was surprised when everyone stayed aboard, except for a few who were going to quit anyway.
You could do what Arlene suggested for a few years. Later you might feel secure enough to require all 45 minute lessons. When you do, you'll love it.
As a side note, I had a piano student quit a few years ago, because her dance studio informed her that she would HAVE to take dance several hours a day, 5 days a week. The studio said that at her level of talent and ability, that was what she had to do. If they can do it in dance, we should be able to do it in piano.

Thanks for the advice ya'll. I may just start telling new people who want to start that I offer 45 minute lessons to all students (except those who I know are too young to go that long... or maybe I know I can't keep their attention span that long). I may just inform parents after the first month of lessons that they're just too advanced and old to have simply 30 minute lessons. Shouldn't be a problem I don't think.
Thanks agaiN!