The Hubble Space Telescope project was started in the late 1970s and it took a dozen years for it to go from the drawing board to orbit, so when the JWST was started in 1995, NASA gave the project a 12 year schedule thinking that they had learned from the HST experience that it would take a dozen years to complete the task.
JWST is now 14 years late, if NASA was still as competent now as it was in the 1970s & 80s then JWST would've launched in dozen years ago and we would already be bored of it and would instead currency be enjoying the recent launch of the 3rd generation of large space observatories.
What went wrong?