Orbital Construction Question

[NewtSoup]

I am experimenting with constructing ships in orbit.

I have no docking ports available yet so I was looking into the practicality of launching individual or groups of parts into space and then using an engineer to assemble a vessel in space.

This was working for me until I dropped a fuel tank and was then unable to pick it up again in construction mode.  I could move everything else on the rig.  Just not the fuel tank floating in space.

Is this because I switched Kerbals mid construction?

I found out that I cannot move the root part of a craft.  What are the other limitations of orbital construction?

[jimmymcgoochie]

I once had a docking port that became a vessel of type Ship when I removed it from a lander and couldn't be EVA construction-ed any more. Changing the vessel type to DroppedPart in the save file fixed that as DroppedParts don't count as root parts.

EVA construction has limits on weight when landed (gravity dependent!) and some parts just can't be used by EVA construction mode at all- in some cases it makes sense as they're just too big and/or massive to reasonably be moved around (e.g. the NERV engine and Mk3 parts) but others aren't so obvious (e.g. the NCS adapter tank seems to be unusable whereas other fuel tanks of a similar size such as the Mk1 fuselage and FL-T400 are fine). They should have a tab in the part description in the VAB/SPH that says if they can be manipulated in EVA construction mode and also if they're able to be put in stock inventory slots plus their volume if so.

[magnemoe]

Think the root node can not be picked up by Kerbals, ran into this using extra-planetary launchpad mod and creating part modules on Minmus. 
Now KAS allows you to move them and attach the root module to other stuff and it can then be moved using the in game construction. 
I say docking is the most practical way for larger stuff but construction is very useful for adding smaller stuff. RCS thrusters is the thing I forget most and its not something you can dock. 
 

[NewtSoup]

Sorry for being slow here and thanks for answering guys.

I have worked it out - Jimmy's answer gave me the correct clue.

The dropped parts had changed vessel type when I moved focus from my engineer doing the building to the ship in order to move a kerbal.

I think to rebuild ships in orbit you have to plan it very carefully.