Are your AMS-FRA and FRA-BRU feeder flights in Economy?

LH does not allow you to pre-assign seats in Economy on intra-European flights unless you are LH*G.

You will however, be able to select a seat via Online Check-In 23 hours prior to the flight, or just get a seat assignment at the airport when you check in.

Exit rows will be tough to get - LH*G (SEN and HON) can get these pre-assigned so they're often taken before OLCI opens.