I had similar problem with CBSA agents, it seems like they are not very familiar about how SuperVisa works. First time they said they didn't know about it and had to analyze it so we waited and waited and finally they stamped the passport without a date. I asked how long we had and he said we had 1 year. Then I asked if it was not 2 years (first SuperVisa stamp) and he said "sure, 1 year, 2 years, does not matter.
Second time they didn't stamp, we asked for a stamp but the agent said we didn't need one. We crossed the border again in less the 6 months and then they said our previous stamp had already expired, we said we were sorry because we thought that it would be valid 6 months after last entry but the CBSA agent said we should have left by the date of the last stamp. He was nice enough to stamp again and put a date 6 months later.
Last time we said my mom would stay 1 year and the agent said we had to go inside to ask for a stamp of more than 6 months. We went inside and the agents didn't know much about the SuperVisa, they discussed among themselves for a long while, then the agent ended up asking lots of annoying questions, gave us one year then changed her mind, invalidated the stamp, and gave us another stamp valid for only 6 months :/
I got tired of these undocumented discussions at the border so I asked CIC for an Extension Letter. Now we got an official letter stating we have 1 more year. I really find it better than crossing the border and relying on what a given CBSA agent knows about the SuperVisa program.