I've bought some gorgeous flat strappy sandals, not the ones in the picture but similar strap style at the back. Problem is the straps at the back of the ankles just won't stay up. It's not a matter of size or fit, making the sandals impossibly tight won't even make the straps stay up. Is there anything I can do? I tried some suede sticky back strips that I bought off eBay and they are rubbish!