Hi, me again... 
First, "latest" as HikaShop version might be true or not, especially these days when 2.3.5 was just released. Not to mention that it won't help anyone else looking this up later. So, thanks for a "correction" there.
Then, and this is surely meant seriously and definitely not sarcastically or so:
With some products showing up in the cart (demo content) and some not ("yours"), you may want take a second look again
and compare:
- What's the message when adding either products to the cart?
- What's the difference regarding key parameters in between demo and "your" products?
Obviously, there is at least one essential difference... maybe leading to a quantity limit being reached, or so. Try to replicate the parameters of a working demo product in one of yours, and see if it works. For the "learning process", eventually do so one parameter at the time, so you'd know which one was the culprit.