Why aren't 2 external monitors working with MBP (mid 2014)?
According to Apple, your MBP supports dual monitors plus the native display:
https://support.apple.com/kb/sp703?locale=en_US
You may have a bad HDMI&gt;DP adapter, but my bigger question is why are you doing that in the first place? Why take Thunderbolt and convert to HDMI, when you have a HDMI port on the Macbook?
I recommend that you connect the HDMI port to the HDMI port on one monitor, and you connect the Thunderbolt to DP on the other monitor. Thunderbolt outputs DP 'natively' so its a better fit for DP.