I went on Wednesday but wasn't successful as they had reached the limit for the day. I went back on Thursday very early in the morning and was lucky to be attended to. I used the Lewiston-Queenston Bridge.
My advice is that you should try to get back to the Canadian border by 8am after flagpoling in the US. If you get there before 8am, you will be turned back and would need to go back to the US border for another flagpole. This happened to me. And if you go late, you might not be lucky to be amongst the few that would be attended to.
Please note that this is applicable to the border I used and I don't know what is obtainable at the other borders.