First thing to do is isolate whether the problem is in the tractor, or the implement. I have no experience with a sickle bar mower at all but have seen a couple and they use the PTO to operate the bar and a hydraulic cylinder to elevate the bar. Ones I have seen are hydraulic powered through remote outlet(s) on the rear of the tractor. If yours is set up like this, plumb a 3000psi pressure gauge into one of the hydraulic remote sockets and pressurize that circuit. I don't know circuit relief pressure on your tractor, but arbitrarily say relief pressure is 1000 psi. You should see that on the gauge, and hear the flow over the relief valve which can be a scream, or a squeal. If unable to reach relief pressure for the circuit under test, the problem likely is in the tractor. It could be a relief valve problem but too preliminary to ascertain anything without testing.

Steering systems are typically operated through a "priority" valve setup meaning the steering system will receive oil under pressure before anything else. Not a lot of pressure required to operate.

That test gauge setup you put together, put that in your toolbox. Handy little thing to have around for quick checks.