Sell Limit Order Issues

Hey Guys, apologies in advance for the most likely Newbie question.

Situation - I am long a well known, and very actively traded LSE Stock. Market is trading at lets say 209.00. I enter an order into the Trading 212 system as a “Sell Limit” order at 210.50.

During the following 10-15 minutes the market moves slightly higher and definitely trades through my limit sell order and trades at 210.60. It closes a few miniutes later at 212.00 for what its worth.

Trading 212 tell me no fill. How can that be? The LSE is an automated system, its not open outcry. If my order is sitting there at 210.50 and a higher price is traded, surely my sell order at a lower level then the market had to be filled?