usually the poly pipe is rather forgiving when you use brass insert fittings. The plastic/poly inserts can cause trouble. Double check that the insert fittings you have are for the type of pipe you are using. Poly pipe uses one type of insert fittings and pex pipe uses another type of insert fittings. Generally poly pipe is used underground whereas pex is used inside buildings. Poly is cold water only. Pex can be used with hot water. Make sure you are using a 160 PSI rated poly pipe with NSF approval for potable water. The irrigation type of poly pipe is usually rated for lower pressure and leach VOCs into the water.
Also I should note, there are two different sizes of poly pipe. The most common is IPS sized pipe, which is what places like menards sales. There is also CTS sized poly pipe which some municipal water systems use for drops.
In a nut shell, you could have 3/4” IPS poly, 3/4”CTS poly or 3/4” pex pipe and they all use different sized fittings.
The insert fittings for poly pipe should be noticeably difficult to get into the pipe, if they just slip in, something is wrong. We usually have to lightly heat the end of the poly with a propane torch to soften it up to get the pipe to slip on the barb.
Pex pipe & fittings on the other hand will just slip together without much force.