we used dry leaves as children to start fires with magnifying glasses.

That is very good that you managed to start a fire with a smallish lens. I won't tell you how many hours I have spent trying. And the sun was sometimes as good as it gets here, temps very hot. However, when it's hot around here it's always extremely humid...if that matters, presume it doesn't help. A very large lens (6-8" D) does make it lots easier, but I'd never carry that. I was sure I had started fires with a small lens when I was a kid, but I guess not, probably just charred stuff. The fresnel was way down on my firestarting list (if it's that sunny, another way should be easier is how I thought) but still pretty frustrating I couldn't do it. I'm sure I haven't given up, the next time I'm sitting around in the sun with a brew...

One of the tricks with lenses for fire starting - continue to adjust focusing all the time. Due to simple single lens aberrations there is no exact focus which you can easily see. So just move the lens up an down a little bit, watching the bright spot (preferably in sun glasses). Change the direction when the Sun image starts growing.

2. With thin tinder (paper, leaves...) avoid burning holes. Gradually move the image of the Sun (the focusing point) to the sides of the burnt spot as it's develops.

3. Bright tinder (paper) will work better if you crumple it first. Also you can use a marker or a pen to darken a spot on it.

4. Make sure the lens is always perpendicular to the direction on the Sun. Watch the defocused Sun's image on the tinder or on your palm first. When the lens is oriented properly all of the image's features will be perfectly round and concentric.

5. The shorter focus (more convex) and the bigger diameter lenses works better.