I've just downloaded 0.19.1, and for some reason it just won't let me attach fuel tanks to decouplers. Have they changed something about the assembly behaviour?
Sadly there is several glaring bugs with the VAB/SPH attachment system.
An due too that area of the game not been under development for 4-6 months (VAB/SPH) The Dev's haven't invested any time, resolving them.
Truthfully I can't imagine they will be priority until the Game goes into Beta Stage, Presently Alpha.
Viable work a rounds:
1. Press Save, an reload the craft, then try to place parts again.
(Works very well when attempting placement of Multiple Symmetrically placed Parts.)
2. Move the offending Part, up the craft tree.
(ie, try placing it on wings or command pods, then removing that specific part, an re-attaching it too the intended place.)
3. Bring up the Debug Menu (ALT + F12) An select No Part Clipping Function.
(Makes craft creation much easier.)