You have answered your own question.
If you move on without knowing something then the results will either be:
(1) You drop marks in the exam on an isolated topic
(2) You move forward with a topic in C1 that appears in C1 with no foundation which has a knock on effect.
Let's say you don't really understand geometric series questions and you are spending silly amounts of time on it. Leave it and employ damage limitation as you are less likely to need this as you are (for example) integration or trig equations.
If you don't master trig in C2 you can't master it in C3,C4 and the A2 further units.
Not everybody is cut out to be an A/B grade student and if you are predicted a C/D perhaps its a case of saying 'this is my level' and how can I consolidate before I set my sights too high.
I 100% believe that C/D grade students can get A/Bs as I see it every year with my guys but if you are racing to 'weigh yourself' and get a B on a past paper rather than establishing key understanding on key topics you are building a house on shoddy foundations.
Trig, Calculus and Algebra skills are a must regardless. From there limit the damage.
I love powerpoints to learn from. Try and find the Chartwell 'Teach A Level Maths' Series.
I think I have 8/9 different ways of learning I offer to pupils. They must decide what works for them and I will support that. Its about finding your way and building that base whilst having realistic expectations.
All the best