What vinyl are you using? I've had some that won't cut no matter what I try. The good oracal stuff makes life easier. Some small designs (like letters) just suck. If you have issues with corners not cutting well, check the cut menu for something labeled ed overcut or something close to that. Extending that by just a fraction of an inch can solve some problems. I like to get settings right with long skinny designs that only take an inch or so of vinyl at a time. Cheaper than screwing up a foot every time you tell it to go.
In addition to being properly inserted, the blade must be locked down into the tool holder. On the Cameo 3 and Portrait 2, push in the lock lever until you see the little circle change to Silhouette blue. If your blade raises slightly when locking into place, open the lock lever and push the blade all the way down again. Hold it into place with one finger while locking the lever with your other hand. To lock the blade on a Cameo 1, Cameo 2, or the original Portrait, turn the black lock lever clockwise until it clicks into place.
Welp, I found my problem with the machine. I now have nothing at all bad to say about it other than I really wish it had WIFI instead of Bluetooth. And that I wish this part was in ANY of the paperwork that came with it.
I still use an older (3-4 year old) Cameo and it works great. All my race decals (have cloned some "official" decals that cost $50+ too, stuff for friends, whatever. 50yd rolls of Oracal 651 are like $12 online. It's paid for itself over and over and the software is easy.
No complaints, it still works great. Made lots of fun stuff....