You may be trying to build a VTOL with an interesting design, but can't shift the CoM of the creation well enough. Here's a little cheat to help you:

PLACING VTOL NOZZLES

VTOL thrust ports get their power from active VTOL engines on the aircraft. This power is shared among all thrust ports on the aircraft.

If you look carefully, there are small points which you can use to identify the middle of the thrust port. Use this imaginary line to help in placement of the thrust port or in adding dead weight.

This is great to know if you have other engines on your aircraft.

Use this to help you place the thrust port on your aircraft. You can fine-nudge it afterwards. Once aligned with the CoM, hide the nozzle with nudging and mirror the thrust port onto the other side.

DUMMY ENGINES

How does this even fly?

The Blasto J15 at the front actually does not produce thrust. All thrust is produced by the VTOL engine and nozzles. The nozzles are placed with the trick shown above. Thus, the J15 is effectively a "dummy engine".

By changing the XML variable powerMultiplier under Engine to "0", you can set an engine to produce no thrust at all. This engine still produces exhaust gases, so it does not give away the fact that the J15 does not produce thrust.

You can use this to build aircraft where the CoM is far (in VTOL terms) from the engines.

Dummy engines need not be actual engines; you can also build a fake engine out of fuselage blocks and inlets. Such engines, however, do not produce exhaust gases (but hey, it could have exhaust hiders like the B-2, right?)

Hope you found this useful. Happy building!