Hi Suzy. I have a Medtronic 640G as well, and I'm very lean. I've used both the Quick-set 90-degree, 6mm cannulas in my buttocks, and the Mio 30 angled, 13mm ones in my abdomen. Both of them can often go wrong, and I find I have to change them several times until I get one that 'feels right'. Alas, in your situation, your sone may not be able to tell what 'feels right'.
If all else fails, you could try to get a different pump from your healthcare team. Medtronic only do their own cannulas, but others which are compatible with a 'luer lock' will allow you to use third-party cannulas. There are some cannulas available that are shorter then 6mm, although I can't remember who makes them now...
Good luck.