Having no experience with flexifoil kites take this with a pinch of salt. Just because your lines are the same length does not mean they are necessarily ideal. For instance your bridal lengths/ pigtails might have stretched or mean the the kite naturally flies a bit depowered.

Lengthen the middle lines/ shorten the outside lines until it flies the way you want it too, if still not enough consider more drastic measures such as actually shortening the lines but you shouldn't need to.

Ideal power for light wind is where the kite will just start to begin drifting backwards oversheeted with the bar all the way in at 12'. Generally this is the most comfortable way to ride kites in light wind. Of course some kites prefer to be more/less oversheeted than others.

Some of the bigger ion3's have needed a little bit of a tune to get the best out of them.

What you need to do.

Set the kite up on 4 lines, in the lower end of the kites wind range (for a 12.5 should be about 15 knots) with the trim strap at full power and the kite above your head gently sheet the bar in. The kite should just start to fly gently backwards. If it does not, then shorten the rear lines on the kite and try again. (on my 16.5 I had to make a new knot 2 inches up the pigtail on each side).

With this setup you will have peak power available on the kite. when flying in light wind you may need an inch or two on the depower strap to avoid stalling the kite.

If the kite is stalling when you are flying then you will need to use the next knot down the pig tail to lengthen the rear lines.

If you are down in Geelong anytime then I can tune it for you!

Going to the fifth line will not help much as it was the length of the length of teh rear pigtails that needed to be tuned.