Although attaining qualifications often leads to higher pay, I wouldnt've thought it was obligatory for your firm to do this. If the extra qualifications mean that your position within the company changes, or your workload increases because of what you can now do, then I think you have a right to ask for a payrise, but it might not always be feasible. You could try hinting that you might be moving onto a job with better prospects, but extra qualifications don't equte experience, and perhaps the previous payrises were just given as an incentive to carry on?