I had the same with back wheel on a Mothercare buggy that was made by Petit Star. Mothercare said they kept spares in to orepair it cos it's a common fault with some type of wheels and it's a spring clip thing that comes out. I've had a wheel come off while i was crossing a road. Once in the middle of town when I had a baby, load of shopping and a big box and was parked really far away. It was a 3 wheeler too so awkward to pus with a wheel off and I felt like an idiot with people looking.
I ended up on the third occassion taking ti back to Mothercare demanding a refund even though it was 6 months old cos it wasn't fit for purpose and after an hour they finally caved and gave me my full money back. The maufacturers shouldn't continue making them with this obvious fault.It's just crazy and they shouldn't be allowed to get away with it.