The buggy is most likely not closed enough to engage the frame release. This could be caused by the following:
1. The front wheel position is incorrect - jbar is not facing out. Fix: put the handle bar in the most upright postion, remove the front wheel & then engage frame release
2. Fabric or items in the buggy are holding frame open. Fix: put weight on the plastic footwell whilst engaging the frame (squeezing the frame closed); if this doesn't work remove the front wheel.