I just did the math. In recent years I have made 58 entries to the USA from Europe. 8 from South America and 8 from Canada. I have come in under the visa waiver, with a visa and with a Green card. I have had my passport stamped on every occasion except 1.
I agree with the other posters. You should not have an issue. In the unlikely event you are asked, just tell the truth "The agent did not stamp my passport" OP will not have a problem.