I was having this same issue with the 1.2mm volcano nozzle.
By configuring the line width to be a bit larger than the nozzle it helps to minimize these gaps.
(im using 1.4mm line width)
also, there is a setting in slic3r under retraction settings called "extra length on restart"
I've found that setting retraction length to 0.1 and extra length to 0.5 helps.
i suspect that the volumetric pressure settings might help with this as well but i havent tried that yet.
i also recommend using at least the 1.3.0 version of slic3r.