dont't worry. Because every child's development is different.
I have a niece who was still struggling to walk at 18months!!!
Now, she, at 8, is jumping and skipping like any other kid... motor skills are perfectly comparable to her peers,so she is not slow at all now.
Some toddlers take more time than others. Its PERFECTLY ALRIGHT.
If you really wanna speed up the training, which is completely not necessary, I only have one word > PRACTISE.
Spend more time with her and let her practise walking by using 'baits'. *smile*