I have a 8 and a half month old bassett/beagle mix who is house broken. She's very well behaved, won't get on the furniture (except when we're not home sometimes she'll sneak a little nap on the bed

), she's eating her normal portions and drinking the normal amount of water. Everything is normal, except that she has heartworms. She's getting treatment in the next two weeks.
Now, here's the problem: She won't do her business in the morning with me! She'll do it at noon, she'll do it in the evening, she won't do it in the morning.
Today I stood there for about 30 mins, till she finally went. When she did go I praised her and she was happy.
The spot where she goes is also used by other dogs...could that be the problem? We don't have a backyard so this is where we take her.
She does go with my fiance in the mornings and up till about two weeks ago she was going with him in the mornings and then I would take her out the rest of the day.
She gets a lot of attention. I've been home with her almost all day except for about three hours. I play with her, give her treats, and always make sure she has plenty of food and such.
So I guess my question is, How do I make my dog go on command? My family's dog goes on command, but we've had him since he was eight weeks old. I just remember telling him to "go pee" and eventually he learned what it meant. Do I do the same thing with her?
Do you think it has anything to do with the heartworms?
Thanks