Just topping up an air con system can be difficult to ensure the correct weight charge of gas in the system.
The way I always do it is using the sight glass in the receiver dryer (if it has one).
I always top up through the low pressure line and ensure the bottle is the correct way up for supplying gas and not liquid into this line.
It is important too to purge the pipe being attached to the bottle and before attached to the car so no air is introduced into the system. With the engine running as gas is being drawn in from the bottle and line pressure increases the clutch should first click in and start the compression and flow.
Watching the dryer sight glass it should start showing bubbles in the liquid and then clear to just liquid. At this point I shut off adding anymore gas into the system.
Obviously lots of problems can cause issues with the system, indeed many cars I have seen the receiver dryers break down and put particles into the liquid which get trapped at the expansion valve.
A lot can often be assessed by feeling the heat/cool of pipes at certain points along the system too.
All these sort of problems do require a complete discharge and any parts replaced, then a vac and hold before putting back the correct charge weight.
I had not realised though later cars had an ecu in the circuit.......glad all my classic cars are pre the voluminous use of ecu's everywhere today. I would rather deal with all the Lucas LRA relays