Later stage: the role of current investor in the next funding round. Can they be showstoppers?

What is the role of the current investor to make a portfolio company ready for a later stage – Series B/C investment? What attitudes are considered added-value for later stage investors? What could be red flag for them?
Experienced international investors with notable track records in later stage investments Chris Church from Kreos Capital and Karol Szubstarski from OTB Ventures shared their opinion at our Equity Thursday online meetup earlier this year.