Like CashsMom, I've never had problems FMQing when I use 505.
That being said ... oh yes I have ... there's been many an operator error!!!
But I can't blame it on the 505. Wish I could!
Some thoughts for you ... when practicing on a small sandwich, it is different than a big one.
First of all, consider is it assembled identical to your "real" quilt,
including the 505, and same batting, and fabrics and treatments of each?
Further, you get more drag the bigger it is, so harder to move around.
The big/small quilts are you moving them the same speed under the needle?
Continuing on with suggestions for you,
you may need to change the type and/or size of needle you are using.
Also, the tension may need to be adjusted.
Just some ideas that may help you trouble shoot!
Good Luck!