It doesn't really make yout blood sugar come down, it stimulates so you release insulin, are you eating a lot of carbohydrates?
I thought metformin decreased the amount of sugar produced by the liver rather than increasing the amount of insulin produced by the body, plus it's supposed to help with weight loss slightly. Anyway, you're right about the carbs
Having said all that, it does make a difference, and is enough for many people, particularly if they also reduce carbs (most important) and exercise more (will also help but really it's all about the carbs).
@Oggykev
If you've reduced the carbs and your levels are still going up there's always the possibility that you are a misdiagnosed T1/LADA (ie your body is not insulin resistant but is in fact gradually ceasing to produce insulin at all). Though LADA is much rarer than T2, many LADAs are initially misdiagnosed as T2, and reducing carb intake can keep their blood sugar levels in line for a long time (because reducing carbs reduces their insulin needs to the level their bodies can cope with).