This is a very slow take. If it is necessary to build up the pressure appreciably like this before the hole will take grout, either the mix is too thick or the cracks are relatively tight - or perhaps both of these.

Did the water test before grouting indicate fairly tight conditions? If it did, these slow takes can be expected on relatively thin mixes such as 3 : 1 or 2 : 1.

When grout has difficulty leaving the hole or penetrating along the cracks, slight closing of the control valve causes immediate response on the pressure gauge.

Is 1 : 1 or thicker in use? If so, it is too thick for this hole. Unfortunately it may not be possible to rectify this by washing the hole and restarting with a thinner mix, because the entrances to cracks will probably have become sealed off.