Nissin i40 and i60a fit problem.
My i40 is similarly tight, but I think it's just the thickness of the flash sole plate, not the lock pin. If I press the release button fully, the exposed pin is hardly visible, and it compresses completely with little pressure.
I found that putting a small amount of silicon or teflon lubricant on the edges of the sole plate (or in the slots of the hot shoe) made insertion/removal much easier...again suggesting that the lock pin is not the problem
Sherms flickr page
|Peruvian sweetness by VickyGo|
from street life
|Floating Dice by TX Photo Doc|
|Nautilus Sliced by Buzz Lightyear|