I'm prepping to fill my first pen and from the videos I've seen, you can use a syringe with the plunger removed OR a luer-lock needle (without the syringe), and even add a filter if you want.
This might be off-topic, but I use a pen/gun to inject from regular syringes. It helps eliminate hesitation during injections, although it's not great for travel or handling. The product I use is the Owen Mumford Autoject 2 (AJ1300/AJ1310), on Amazon for around $45.
Does a pen exist where you click ONCE after dialing in the units? Dial in 10mg, hit the button, and it injects that amount. Doing all these clicks is a problem…
This pen lets you insert a syringe with your medication and uses a spring to inject. My wife has MS and used an older model with her medication. It works great and lasts for years. You just need syringes and alcohol wipes. https://www.amazon.com
I found this useful, too ScaleHater84. It's not as compact, but using a standard syringe can be really helpful for some. It makes sense for those still getting compounded meds in syringes.
The Owen Mumford pen is spring-loaded and uses regular 3ml carts. Dial in the dose, compress the spring, then press the release. It's cheaper, and the dial isn't as good as Chinese pens, but I like not pushing the plunger.
It could be the pen or the cart. Try the pen without a cart and see the resistance. If it's okay, you might need a different vendor or the cart's internal plunger isn't installed properly.