Why this change in morphology in endothelial colony forming cells (ECFC)?

Dear colleagues, I culture on 2% PAN-biotech gelatine, cord blood MNCs into endothelial colony forming cells (ECFCs), using Endopan 3 kit medium. 

For many of my samples, I obtain a change in morphology of my ECFCs during culture, and these "new looking"  ECFCs no longer proliferate. The cells now look enlarged , elongated and lose their cobble-stone morphology as seen in the attached picture below. 

I thank you in advance for any suggestions as to why this happens and how I can help the situation.