Sforno on Exodus 6:9/ספורנו על שמות ו׳:ט׳

1ולא שמעו אל משה, to mentally absorb all this, so that they would have full trust in the salvation of G’d and give Him credit for this. This is where they were different from Avraham in his time, who believed implicitly in an impossible-sounding promise, and who gave credit to G’d for making such a promise. This was eventually the reason why this part of G’d’s promise was not fulfilled in the lifetime of the people who were being addressed at this time.

2מקצר רוח, for it did not appear believable to their present state of mind, so that their heart could not assimilate such a promise.

3ומעבודה קשה, if it had not been for the heavy labour which they had to perform they would have paid much more attention to Moses’ words, and they would have understood that Moses’ arguments made what he said very believable.