Oxtail soup - oxtail tightened up during simmering? I've been hungry for a light oxtail soup, and managed to find some (around 1.5lb) frozen at the butcher. I'm aiming for light flavor to go alongside leeks and bulgar, so I figured I would simmer the oxtails until they fall apart, pull out the bones and simmering veggies, and then add sweated leeks and bulgar and reheat.

I let them thaw for 2 days, and then earlier today put them on a baking sheet for 10 minutes at 400 degrees, turned, and did another 10 minutes to lightly brown.

Afterwards I put them in a slow cooker with some veggies, 4 cups of water, 2 cups of beef bullion, and 1 cup of cream sherry, and set it to a low heat. to start simmering. I am planning to simmer for around 20 hours, but at about 5 hours in the oxtail chunks are still very tight - think the texture of a steak that's been cooked, put in a refrigerator and then reheated. I know I have another 15 hours to go, but still, I would have thought I'd see it breaking down a little bit more by now. I didn't try and slice off any of the meat from the bone, so maybe that is it?

Did I do something wrong, or am I jumping the gun timewise? Candid feedback is appreciated!
A common braising time for oxtails would be on the order of 2-4 hours.

Oxtails are a tough cut of meat, with a lot of connective tissue made of a protein called collagen.  It needs to cook in a hot, moist environment in order to hydrate into gelatin.

This process happens over time and temperature.  Typically, braising is on the order of 200 degrees F (93 C), give or take, in order to get the expected tender product in the expected time.

If your oxtails have not become tender already in the 5 hours you have already given them, perhaps your slow cooker's low setting is not hot enough.  This is of special concern because you also want it to be at least 140 F (60 C) for safety.

I strongly urge you to use an instant read thermometer and measure the temperature at several places within the pot.  If it is not above the safety threshold, you should discard the contents, which would have been essentially incubating for the entire time.

If it is lower than the desired braising temperature, you can try your high setting, or transfer your braise to the oven (if the crock is oven safe, or in another covered oven safe pot such as a dutch oven).  Oven braising is usually done at about 350 F (180 C), which should bring the contents up to the desired braising temperature.

In any case, an expected cook time of 20 hours seems excessive by a factor of about 4 times.

Note also:  if your goal is to produce beef stock for the soup rather than tender, delectable meat, you want the temperature up around a slow simmer, which is about 200 F (93 C), although at the end, the flavor will have gone from the meat into the broth.  Even for stock making, I cannot imagine a cook time being required in excess of more than 8-12 hours.