I have been using an Anycubic i3 Mega for about a month now and it worked fine. But when I designed a simple model in Fusion 360 and sliced it with Cura it started to have issues sticking to the bed. I thought the problem was the model so I tried to print a Benchy, but the same problem occurred. I readjusted the bed but it's still not working. I'm not sure how I can trouble shoot this.
The proper answer could be not related to Fusion, as you already noticed.
I suppose the issue is that the Ultrabase surface (or any other glass surface you have installed) was not cleaned during the first month? So it should be cleaned from grease and possibly dust. The general cleaning method is to use isopropyl alcohol (IPA) from time to time (btw. I am curious if it is advised in a printer's manual). If it does not fully help, I suppose using a dish soap with hot water prior to IPA is good step. Though I do not own this bed, so could not be 100 % sure of impact. (Definitely do not use any extra adhesion helpers for Ultrabed, because it has some specific microstructure which will get stuffed.)
You could want review other threads like this How to get Sunlu PLA to adhere to the printing bed? to get other hints and supplement your checklist. One thing to check first (and regularly) is proper bed levelling at current moment - and compare to initial layer settings in Cura (thickness, width, flow percentage).
Keeping the print bed clean as mention in other answers is key. Dish soap (without any additives) and water to wash away oils from your hands the printer itself is the first step. Isopropyl Alcohol (IPA) is also good to keep the bed clean between prints, but when you run into adhesion problems, wash with soap first. IPA in many cases just move the oils around the bed instead of removing them. Also, always wipe from the centre of your bed to the outside.
The next step is to also ensure the bed is level (unless you have auto-levelling). If the bed is level, then you need to make sure your z-offset is correct. The first layer must be printed with the nozzle low enough to press the first layer into an oval shape onto the bed so that the surface area in contact with the bed is increased. If the z-offset is too high the first layer filament is going to have a round shape which means the printer is basically printing in the air.
Be careful not to decrease the z-offset too drastically because the nozzle should never come in contact with the bed because damage to the bed, nozzle or heat-break is possible.
A "hack" I used was to print with a raft. I decreased the width of the raft in Cura so I'm not printing with excess plastic and the raft ensures my print always sticks.
Another thing to look at is to look at the print speed, a slightly slower speed might help with the build plate adhesion. This is something I noticed from printing with a raft
For reference, I print with a glass bed and the plastic failed to adhere to the bed even after washing it and using sanitizer to clean it off. IPA is the real deal though but I didn't give it a try.