Shower Spa water flow problem

We have a Shower Spa with 5 jets, a Rain shower head and a handheld shower and a small spout. There are 4 control valves, so one for each feature.
We have issues with the water flow.
When we first turn on the spout (located closest to floor), there is relatively good waterflow for about 2 seconds, then it changes to a trickle.
The body jets and the rain shower head have no flow at all, the handheld only gives a few drops of water.
We build a new house and all other faucettes in the house work fine.
We bought this unit from Costco and have no idea about the brandname.

What do you suggest we can do?
Has anyone heard of similiar problems?

Thanks Romy

The most common cause would be a loss of pressure in either the hot or cold lines, but we cannot tell what could cause it. A stuck balancing spool would stop either the hot OR cold, but not both.

There could be something caught in the valve(s) that floats to block flow once you open it. It could be a glob of solder, dirt, a pebble, a flap of teflon tape, packing material, etc.