How to fix the curve of the leg flipping up

I'm in the process of sewing up the first toiles of the patterns that will available to buy in November this year (2016). I finished #2 pattern The Penelope, a tanga with fold-over elastic, and the curve of the front leg of the elastic, instead of sitting flat, stood up vertically, perpendicular to the brief. 

Has this ever happened to you? And do you know how to fix it? There is nothing worse than having spent all that time and effort sewing up a pair of briefs, for there to be an error and not know the reason why.

On this particular brief, it was all down to the elastic, it was too heavy and it didn't stretch very much. It was the type of elastic that would be best used on the top wing of the bra or maybe even the top of the bra cup.

As you stretch the elastic when stitching around the legs if it's too heavy it takes it's original form, it's not soft enough to sit flat. 

Other reasons that the elastic may flip up:

1. Would be if the curve of the leg is too sharp, so you would need to make the leg curve shallower.

2. The elastic is too wide so the elastic can't sit flat around the curve, so the only way it can sit is if it flips up.

I hope that helps? Also if you are new to elastic and can't gauge if an elastic is heavy or not, hold it up to the light and stretch it, the lighter it is then more light will shine through it. So a dense heavy elastic you won;t be able to see through it.