My DD2 is 16months today and has walked on her knees for months!!!
She would walk along the furniture, push her baby's pram for a little bit and sometimes walk holding our hand but wasn't overly interested. All of a sudden she just wanted to walk everywhere! We have to hold her hand but she cracks a mega tantrum if we carry her and she wants to walk. She'll come up to us, grab our hand and say 'walk'. Now, she's starting to let go of the lounge and walk a bit. She's doing so much better in basically one day! Of course, she still walks on her knees most of the time tongue

Don't worry about it, just encourage her to walk while holding your hand or push a dolls pram etc. She'll take off when you least expect it smile