What maths textbook do you use?
This feeling sounds familiar. It is either the problem of you using a very easy textbook (eg Maths in Focus, or Jones and couchman) which do not offer a diverse range of questions to challenge your understanding of the theory OR you are not learning the theory properly. But I am leaning towards the former. The exact same thing happened to me when I was using MIF and realised that surely high school maths can't be that easy. It wasn't until I opened up Cambridge and Fitzpatrick that I realised there's so much more to maths, than just MIF and its band 2 questions. If this is the case, take immediate action and switch to Cambridge. If not, then forget what I said and you may need to dedicate more time to maths/get a tutor.