I have a sub panel about 200’ from my main that is fed by MHF buried, not in conduit. The sub panel just runs my well pump at 220v 10a and a 120v outlet. Everything worked fine for a month and then out of the blue, the well pump wouldn’t run. So I measured the voltages at both panels. Going out of main panel I get 125v on both hot legs. Coming in the sub I get L1:124, L2:75, that’s across neutral or ground. L1+L2: 199. The line has been in the ground about 4 months, only hooked up for a month. Seems to me a bad hot leg?
You might be learning why some of us prefer conduit, even for "direct burial rated" wire. The damage protection looks inexpensive the second time you have to dig the same trench.
One further check would be to plug in a load (like a plug-in heat appliance - heater, waffle iron, hair dryer...) to the outlet at the far end and check the voltage then. Switch the outlet to the other hot line and recheck.
But yes, it sounds like a bad connection in a hot line. Start by re-checking and re-torquing the actual connections (terminals) at both ends, you might get lucky and not have to dig it up. You did use proper aluminum anti-oxidant paste on those connections?
Something else to check B4 replacing wire.... Go back to main panel, and check all voltages, L-L, L-G, L-N, N-G, at every point you can access. (Bussing, incoming feeders B4 main breaker if possible, load side of several breakers, at ground buss and directly on Grounding Electrode Conductor, as well as neutral buss(most likely common with ground buss if it is service panel) and at closest possible point to utility neutral conductor connection.) Check with all loads disconnected and record your exact voltages for L-L, L-N, L-G. Both L-N readings should be within 0.1 - 0.3 of equal and add up to your L-L voltage.
Now, turn on all your 120v loads one one phase only. Again check all voltages, then turn off and connect all 120v loads on other phase and check voltages. And finally, turn on everything and be sure a 240v load is in use (a\c, dryer, well pump, etc.) And again check voltages.There should be no more than an insignificant variance from initial readings.
But, if you notice that voltage drops by more than a volt or 2 on one leg at any time, then you must likely have an issue with your neutral-ground system. If you disconnect all 240v loads and turn on everything you reasonably can that's 120v, you can get HUGE imbalances on the 2 hot legs. Worst I've personally seen is 189 \ 31 split on the phases. If you see something like this ( even just a couple volts rise\drop) then turn on a 240v load and your L-N voltage will more or less balance again while that 240 load is running.
If you see anything other than very stable voltage no matter what combo of loads are running, you will need to check your grounding and bonding. You will find a bad connection somewhere in your incoming neutral \ grounding electrode bonding.
This imbalance gets worse the further away from the transformer and your grounding system you get. So what may only be a few volts at your main panel can be many multiples that another 200 feet away. I live and work in n.w. Arizona where there are 1000s of OLD, pre-code enforcement services that are heavily loaded running air conditioners. This is a quite common issue to run into here. If you find any voltage fluctuations, LMK and I can help narrow down exactly where to look for problems.