Renault Scenic automatic gears slipping

When sitting in traffic sometimes the gears don't work and the car lurches forward. Bought some stuff from USA which they use in the auto boxes and it should have been quite an easy job - drain one and put the other in. However, when getting car checked out it appears it has a sealed unit and doesn't appear to have a dip stick (apart from me it seems).

Can anyone please help with this problem? Is there a way around it?

Any help you can provide would really be appreciated as I'm now tearing my hair out!!!