Hi there..... at times I deal with the snug fit of those fittings.

I normally prefer to use the good ole' primer and PVC glue with SLIp to SLIPs, but when it comes to the fitting you speak of... take a little petroleum jelly to lube the inside of the flex pvc. Start to twist in the barbed fitting, and it should start to work it's way in.

I've used a soft mallet to further pound that barbed fitting all the way on.

A hair dryer to help soften the flex tubing will also allow the barbed fitting to slip in easier.
My Ponds ---