I think the thing that helped the most is we built great relationships with our small clients. We convinced them to let us observe their day to day, speak to staff etc. From that we got an intimate idea of all their biggest pain points and we were able to offer complimentary products that worked with their existing core banking (and abstracted it wherever possible). Things like secure communications, document management, form builders - things we were planning on building after our core banking but were actually a bigger pain point for them. It made such a difference to the actual staff that they would be bragging about us at conferences and networking events. Eventually the word spread in the community/industry and soon enough the big clients went from ignoring our calls to asking for "exactly what they have" (referring to the small clients who took the risk innovating).

TLDR: Try starting with easier to sell, complimentary products. Focus on the smaller clients as they're more willing to innovate. Do such a good job that they can't help but brag.