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This game can just be very unlucky unfortunately.  Obviously being based off of the original FNaF, Fuzzboobs works in the exact same way with some minor tweaks, and FNaF itself is very RNG based in some regard.

If you want a more in-depth explanation for why this is, watch Tech Rules' video on dissecting Five Nights At Freddy's' AI (as it mostly transfers over to this game too- I think).

All in all, this isn't a flawless guide- it can't be, since everyone adopts different strategies. Especially for a night that's actually difficult like Night 6 or 4/20, where you might need to take risks to conserve power, different people will do different things.

There's probably better guides out there somewhere in all the posts, but either way it goes, the issues you're finding with Bonnie & Foxy are pretty consistent with what I experienced in my playthrough as well, and I'm pretty confident it's likely just bad luck.

For Bonnie, don't use the lights at first. Hear her  arrive and just shut the door. Leave it for 3-5 seconds and flash the light on the door. If she's not gone, don't keep flashing, wait 3-5 seconds again.

For Foxy, she's an asshole. She can arrive whenever she wants really, and checking on her is only necessary because if you don't, she will arrive quicker on average. In the video I've referenced from Tech Rules, seeing how Foxy's system works makes it clear why this is the case.

In actuality, this guide may be useless in regards to Night 6 because my notes on Bonnie & Chica are made considering that in 4/20 mode, Bonnie & Chica actually leave the quickest out of any of the nights. If you look at Night 1, Bonnie & Chica stay at the door for ages, and this is because their low AI difficulty stops them from moving quickly. In 4/20, they're at max speed, so they arrive often but disappear in seconds. 

In Night 6 however, they are middle of the road at around 10 AI difficulty out of 20. In fact, I checked the video and these are the AI statistics:

[Bonnie = 10, Chica = 12, Foxy = 16] This would actually explain why you are having troubles with Bonnie in particular and not Chica. Bonnie's AI difficulty is lower than Chica so she'll arrive and stay longer before leaving.

As '20' is just the other way of saying '100% chance to move at every opportunity', Bonnie being at '10' halves this to a 50% chance. This means every few seconds, you're getting a 50% chance for her to leave the door- in your case, Bonnie keeps failing this 50% chance and stays at the door. That's what makes Night 6 so infuriating, as an AI level 10 can act similarly to a '20' or '1' AI interchangeably due to the chance for leaning towards one or the other being only 50%.

I'll stop writing now, as not everyone likes a long-winded response, and I'll cut this short. I highly recommend the video if you're looking to try beat the game and this guide isn't flawless, even with luck factored in. 

{Since they could've re-worked the game to their own system, the video may be redundant but I highly doubt this is the case, I'm unsure though}

Hope this wasn't too tedious to read :/
Good luck.