I have been fixing a used NS Oscar and have had issues with water flow from the group head. It was originally a direct connect machine but I have converted it bank to a tank. The water comes out in spurts or sometimes not at all. I have cleaned the solenoid valve by the group head as well as replaced a retention valve to the boiler. I have read that these are two common problems with the machine, but they haven't fixed the problem. Steam is fine and the pump turn on. Any thoughts?

ya probably already know

well it super easy to destroy the neplex valve on installation, equally easy to install backwards.
the simple test is to remove the ptfe hx feed from the double union and blow, this will clear the line the the 1way neplex. then apply suction, if the neplex is intact, no flow.
have found remnants of the neplex in the upper hx feed line, the grouphead, outside and inside of the 3way

I just replaced the valve but suspect I didn't quite do it right. I haven't seen a lot of information out there on how to install it, but I think it may have been seated to high when i tried to put everything back together. I put it in with the black rubber facing away from me which I believe is the correct orientation. If I try to run the machine with no valve installed, I still have the same issue. I have tried to clear out the area on the right side of the grouphead, but also haven't been able to the copper plate that is attached to it. I have turned the pump on with the solenoid disassembled and water spurts through it. When I was connected to a water source, water would freely flow out even if the pump machine was off.

retaining valve green side up
locating depends on what was disassembled
if assembly is removed from T fitting and locknut, push as far up as possible
if assembly is separated at the ptfe tube and nipple side, as far down as possible
in either case thread sealant or thread locker is required

when plumbed in line pressure will push thru the vibe pump and charge hydraulic circuit, so constant flow is expected

have you disassembled the 3way?

I have removed the 3 way from the group head, but haven't been able to fully disassemble it so far. I am fairly certain my orientation was correct. The water should be able to flow through the hx, but not back towards the pump. I totally trashed the valve so will have to wait a few days for a replacement. I'd love one of the all metal valves, but hesitant to wait for it to come from Italy.

A little bit of an update. I reconnected it to a water source and everything flows fine with the solenoid valve off. When I put the valve back on and tried to run the machine, still not a lot came out, so I'm thinking the problem is probably something to do with the group head itself or with the 3 way valve

When I turn the machine on with the tank connected, water will start to come out of the group, but at a much slower rate than seems correct. Eventually, that flow turns into a dribble and a sputter of steam. I will have to reinstall the retention valve when the replacement comes in the mail, but I know this is likely mostly to do with that. Also, the tube the carries the water from the pump to the rest of the system seems to be rattling a bit. I've been trying to disassemble the solenoid valve, but it is stuck pretty tight and I have no idea how I am going to go about getting it apart.

I got the retaining valve installed and now water is not flowing backwards from the HX. There is still a lot of steam that comes out of the group after some time. Every time I take off the 3 way, there is some scale blocking the hole. could the scale that is breaking loose in the machine be the issue?

yes
scale
if you have no vice
you can break loose the body, from the base of the 3way when it's installed on the grouphead

I finally got the 3 way taken apart, and there didn't seem to be anything obstructing it from moving freely. I may try to descale the whole boiler in a small bucket to avoid getting water on the external parts of the heating element.

I'm thinking there also might be an issue with the pump. When I was using the direct connect line, hot water was able to flow out through the back of the machine.