I taught my class last Friday on this theme
What to do when technique fails?
99% of the time it's simply because you didn't take uke's balance.
There is a large bag of Aikido 'tools' available to us to enable us to take uke's balance, and you just have to learn what each one is and how to use it (appropriately).
I'll try to get a class synopsis onto my blog at some point soon, rather than going into depth about it here
However, IMO large folk who resist are an absolute gift, as I can try out all my Aikido tools on them and see which ones need sharpening