Poundland puncture repair kits contain a 'cheese grater' that levels the seam flat after a few strokes. It's then easy to get an airtight seal with the patch.Some tubes have a pronounced seam on them - I have found that repairs to holes on the worst parts of this seam can be almost impossible to repair properly
Think the tube on my commuter has at least 6 patches.
I've had more than that before.
@SkipdiverJohn may have a view.