For blend-in to work, you need to have make sure that only one is playing at a time. For example, if I am transitioning from walk to run, the walk action actuator needs to be turned off as the run actuator is turned on.
The “Play” mode for action actuators is not ideal for blend-in, but they should work (As long as you aren’t using a “Play” mode as a “Loop End”, where the animation keeps playing over and over because a True pulse is being sent over and over.
Usually, when animations don’t get to play fully is because they are were interrupted before (this is from personal experience). Sometimes when you are playing an animation and you interrupt it with another, the first one can start to skip when you use it afterwards. Once again, this is from personal experience, it may just be coincidence.
I’m not exactly sure how the animations in blender work but I usually have no trouble after I make sure that only one animation is playing at a time… Unless I intentional put an animation over another.