My son didn't start walking on his own til he was 19 months old. I worried and fretted too. I just kept encouraging him but didn't force him. I'd already learned that my baby did things in his own time when HE decided he was ready. He too was delayed in all his previous milestones, and still is.
I watched him and noticed that when he would cruise around, holding onto furniture, he did fine but if you tried to make him step away, he'd sit right down. If you tried holding his hands and let go, same thing.
I quickly realized that he had the ability and the mechanics down, he just hadn't DECIDED that he was ready.
One day though he finally did decide and voila, you can't keep that kid still! He went straight from crawling to cruising to RUNNING!
Watch your little girl. Wobbliness aside, does she seem to have the balance? Does she cruise around, holding onto things to travel, but just refuse to let go? Were her previous milestones similar where it seemed she was completely able, just didn't DO til she made up her mind to?
In my case, I was told that he'd been exposed to CMV, a virus like the cold, while I was pregnant, and that it might cause delays, but he is smart as a whip, just stubborn. So I just practice patience with my little guy and hope for the best and just try to be as encouraging as possible.
His speech was delayed also, and he still has trouble with a few sounds but he's talking up a storm too.