This advice has been given a few times on AAM:
"For best results, fill the bath with water before you apply the silicone, as some baths give slightly when they are in use. If you seal the bath without water in it, then when you use the bath, it may stretch the silicone and eventually lose its seal".
I find it best to remove the old silicone with a Stanley knife and start afresh. Of late we have been using the clear silicone rather than the white finish one but it appears to lose its colour over time and doesn't look the best.
Another problem can be that the water travels along a tiny channel possibly behind the tiles so where you are pumping the silicone on may not actually be where the leak is.
Presumably your tiler fitted the edging strip and this helps the water flow back into the bath rather than behind the bath.