Hi Danni. You can fall pg on implanon. Like you can with pretty much any form of contraception. Nothing is 100%.
As Starrysky said, it has to be inserted right. You should be able to feel it. I had a friend whose was in too deep.
I don't think it has much to do with weather or not you bleed while you have it in. Every one is different, some get AF more, some less, some not at all, some stay the same.
I hope you aren't pg yet Do a test to put your mind at ease.