Especially if you pretty much never change your tactics, this seems to be the reason then.
Bottom table teams will play very defensively against you, especially while you are one of the top teams of the league and apparently your tactic is not well suited against that style of play.
In the TT&F thread in the tactics forum by wwfan you will find very good advice on how to adjust your tactics a bit for these games. It's a little annoying having to change something all the time in FM08 but it works wonders if done properly