Welcome back! Patience is always a part of this hobby.
The nuts must be the same thread as the U-bolts. The locking nuts normally will start easily then get harder to turn as the locking feature engages, unless you try to put the nuts on "backwards" and try to thread them on locking end first (please don't ask me how many times I did this before I learned it
With a U-bolt removed and in good light, check the bolt and nut threads to see if they are the same. If the threads don't match or if you can't tell or check them with other bolts and nuts or dies and taps, then you probably need to purchase different nuts. Take the U-bolt with you to the store so you can check the nuts by threading them onto the U-bolt before you buy them.
If the threads are different and you cannot find a locknut with the proper threads you can always use a normal nut and lockwasher instead of the locking nut (that's what I have on mine and I've never had an issue with anything coming loose).
Alternatively, you can buy the U-bolts at most auto parts stores and they come prepackaged with nuts and lockwashers. They're actually quite inexpensive.