D.B.
First off, don't blame yourself for having a single child! I have a singleton too, and he spent a lot of time around adults. In the long run, he wound up being incredibly good with a lot of different people.
It sounds like your daughter has plenty of exposure to other children. You could add in a play date on the weekend if you want - I'm not a big proponent of putting kids in a million structured activities anyway, and it sounds like she would benefit from less of that and more of the regular socializing. I would mix this up with a variety of children, and then you can observe any problems for yourself, seeing what is common from one visit to the next and what issues are present regardless of who the other kid is. Take her to playgrounds while the weather is still good, and let her interact with other kids on a non-structured basis. Let her practice skills to get better at these things!
I would try to get more info from the teachers - is there a particular thing that is difficult for her? Is that a skill area they can help her work on? Does she show delays in any other areas? If so, what services would be beneficial.
If she is just immature, then give her some time! She will grow out of it. Do not be afraid to keep her in preschool for another year rather than sending her to kindergarten. This is what we did with our son - he started when he was 6 and he never knew the difference! It's much easier and much kinder than having to have a child repeat a grade later on when the problems surface. My brother was the opposite - youngest child in his class - and it was a huge error.
Meantime, continue to provide her with opportunities to develop the skills she needs to adapt to new situations and new kids.
A few "well-meaning" acquaintances gave us a hard time about not starting our son in kindergarten at the earliest possible opportunity. "Isn't he bright enough?" and "Aren't you afraid of holding him back?" were the kinder comments. We didn't question our decision then, and now it is so incredibly clear that we did the right thing. Every child develops different skills at different rates - some walk at 8 months and some at 14 months, some talk early and others talk late, some take forever to potty train, etc. And some take a while to mature.
Enjoy her growing up years!