Hi. Welcome to JA.
The sealing of the upper spray arm should not change due to an impropper fit at the connection. The basket will not allow the door to close properly unless the connection is correct.
The only thing that cpould effect the cleaning ability of the upper spray arm is if the sprayer where it connects is worn out. It can cause a low pressure condition as all the water leaks out at the swivel point. It can also cause the arm to not turn. The arm not turnning is the most likely problem as far as the arm goes. This will be noticeable as there will be considerable slop at the connection point.
If the spray arm is ok, then the most likely problem will be a damaged or worn out impellar. This repair is not one that I recommend a novice DIYer attempt. It involves considerable disassembly of the unit.
I have posted a picture of the trouble shooting chart for your unit below.
Please let me know if you require further information.
Ok. If you felt resistance in the impeller and then it turned freely after the first turn or so then you may still have a problem with the impeller or the motor that drives it.
The resistance of the impeller should be consistent at any point in either direction.
Try this; try spinning the impeller both directions. Spin it more than a few times all the way around, both ways. Do that a couple times to be sure that it doesn't again become harder to turn.
If it does then again further disassembly will be needed to determine what is causing the resistance.
If the motor is having to drive the impeller with resistance applied to it then the lower speed of the impeller turning will greatly reduce the amount of water pressure sent to the spray arms. This will seriously impact the ability of the unit to clean the table ware on the top rack while still doing a good job on the lower rack.
If it checks out as good then run the unit for as long as it takes to fill it and begin the wash cycle. Stop the cycle and set it to drain the water and turn off. This will warm up the components and may show something.
Once again check the impeller. If it is free then it should be ok. The first time may have been a coincidence. If you again feel resistance and then a freeing of the impeller something isn't right. Further disassembly will be needed to accurately diagnose the unit.
Please let me know what you come up with.