I am just trying to connect an 1/2” vinyl tubing (this one) to a smaller 3/8” version.
I got this 1/2” to 3/8” barb reducer but it’s way too small and doesn’t fit.
Why?
Which part do I need instead?
I am just trying to connect an 1/2” vinyl tubing (this one) to a smaller 3/8” version.
I got this 1/2” to 3/8” barb reducer but it’s way too small and doesn’t fit.
Why?
Which part do I need instead?
The device you have linked to is for PEX pipe, which needs special crimpers. It is not deigned for vinyl tubing.
You need one of these:
barb hose reducer

(Admittedly, the link is for a set of 25, but a single wuold surely be obtainable.)
You will also need hose clamps.
In my experience (as a brewer), when mating a barb to tubing and everything is sized properly, it takes some significant effort to get the tubing on the barb. This is by design, so that you get a leak-free seal (usually in conjunction with a hose clamp).
Some strategies that might be helpful:
If I ever need to remove the tubing from a barb, I am almost never successful in pulling it off. I usually end up having to cut the tubing off of the barb, losing an inch of tubing in the process.