i bought the same URGI MWS and did very similar mods and had the exact same problems.
it is because the hop-up adjuster is moving and therefore getting caught on the bolt as it fires forward for each shot. This then turns down the hop wheel and your bb’s gradually go lower and lower.
As another user has posted, you need to re secure the screw that holds the hop-up adjuster wheel. It’s where the gas block tube starts inside the upper receiver.
I thought I had broken this part when taking apart the URGI to change the hop-up rubber and bought the Guns Modify metal hop chamber and hop adjuster. However i wouldn’t spend £60 until you’ve tried the above.