Hello all!
I’m new here, but plan to be around a lot! So, please be nice! 
Anyway… As the title suggests, I’ve gone and done something to make the icons I have in Plank invisible and I cannot make them reappear! For a little background, I am currently having this problem on Ubuntu MATE running on a Raspberry Pi 3.
I just started using Ubuntu MATE a day or two ago, and have been busy moving over files and customizing it’s appearance. All I did was change the window theme, changed wallpaper, added Plank to my start-up programs, and set it up to use the GTK theme.
When I booted up this morning, all my icons were gone! I can mouse over where they would appear on the screen, but all I see are their titles, and not the icons themselves.
I also run Plank on my Xubuntu Machine (Which uses XFCE) and cannot replicate this problem.
I have tried removing and reinstalling Plank to no avail. I’ve gone through all the settings and I even tried to use Docky (Which is more advances and derived from Plank) which had invisible icons too! This has me rather stumped, so if you have some ideas, please feel free to share! Thank you all in advance, and hope to hear from you soon!
UPDATE:
I took a few more shots at solving this problem on my own. Out of curiosity I tried running Plank from terminal and got some error messages that seemed to indicate a conflict between the GTK themes of Plank and my desktop. I decided to run an experiment and see if I could get Plank running on Raspian to see if I could replicate and track down the problem without the possibility of screwing up my Xubuntu machine. While swapping out SD cards I discovered that I somehow I got my cards mixed up, and installed Ubuntu Mate to one of my smaller cards and not the 64gb card I had intended to use! D’oh! So, I spent my morning reinstalling the whole system to the larger card, and have everything up and running again. Plank seems to be behaving itself! I have it set to the Matte them for now, and there seem to be no conflicts or other problems. So, I guess when all else fails, chuck it all and start again!
