Bottom Knots???

[deloid]

 I just had the sudden appearance of bottom knots with notably increased bobbin thread tension (by feel). The machine has been perfect regarding tension for the past three years. It is used pretty heavily but well maintained.
Cobra 4- 138 top/bottom
Threading checked and redone just in case.
No tension changes made in the last two days of work on the same material
Thread OK...bobbin clean- no nests of thread. Bobbin tension spring clear of debris.
Timing is perfect.
I can easily understand decreasing bobbin tension with time but increased?

I increased the top tension and the knots raised but now the leather is being compressed too much.

Then I backed off the top tension to it’s starting point (for today) and I kept the bobbin tension looser than it has ever been and still getting bottom knots.
Really need help/ideas. Only two sewing days before an out of state art festival.

[Wizcrafts]

Start your troubleshooting by winding a new bobbin load, using the best thread tension to get tight windings.

Pull out the misbehaving bobbin and install the new one. If the problem goes away, check the windings on the old bobbin to see if they are cross-wound, or if the starting thread stub is sticking out of a hole or from the windings, causing intermittent binding. Also, check the ends of the bobbin to see if they are bent or otherwise damaged.

If there is no improvement with a fresh bobbin, remove the race cover and clean out the surfaces and the shuttle (be careful with the shuttle). It's possible you'll find a thread stub in the race.

As a last resort, reverse the direction of feed of the bobbin inside the bobbin case. This will lighten the tension if the thread is feeding inline with the thread slot, instead of against it.