Won't come immediatly when in park

Hi All, We'll we've been doing School for the last 6 months and Zephyr has mastered all the common commands, and is so good that we occasionally get put in the front of the class to show how it's done, (Proud mum that I am).
At home we train every day and she is perfect. She will "come" instantly from anywhere in the yard to me, or anywhere in the house to me.
My problem is when we are out in the park or beach, I have just started to let her off lead, and as practice I do the "come" command, she won't instantly respond, keeps sniffing and after 30 sec or so, goes ok, I'll come.
I carry treats with me, but she is not interested, and I make sure that the "come" command does not involve putting her back on lead. So she will eventually come and get a pat and off she goes again, I call "come" again a while later and same thing again.
How do I get the same response out in the park as I do at home?

Thanks in advance for any advice

Go to your local feed store, or wherever you would go to buy horse supplies and get yourself a 30 foot lunge line. Until that come is solid, I would keep her on that instead of loose. Every time she is able to wait that couple of seconds, she is learning that she can get away with it. With a lunge line, she has freedom, but you are able to correct her if she decides she is not ready to listen to you.

That's exactly what Wilson used to do! And I totally agree with LavendaRott, you need to put her on a lunge line, or a retractable lead, and whenever you call her to come and she doesn't, tug on the lead to bring her in to you.