when you say regular black, do you mean regular black cartridge? If so then you might be trying to put the photo black in a spot that is supposed to be for just the regular black cartridge area, there are 2 black cartridges, Photo Black ink and Black ink. Do not mix these cartridges up. In fact, the regular black ink cartridge is bigger than the photo black ink cartridge, so it is hard to get them mixed up. But the photo black ink is about the same wieth as the other colored cartridges. But the regular black in is wider than the other cartridges. Even though it has the same capacity size as the other cartridges.
I don't know the setup for the XP-620 I have the XP-810 and it has spots for both cartridges, one for the photo black ink and one for the black ink. If your printer has both save this for the other spots when that ink runs out. If you only have one black ink area on your printer, then you need to get the right cartridge as you are trying to put the special PHOTO BLACK Ink cartridge in the BLACK Ink Cartridge area, It will not fit, not matter which capacity cartridge you try.