First thing is wondering why you transcoded the MP4?
And I presume the footage is on a directly connected internal hard-drive, you navigated to it with the Media panel, then imported it into the Project panel, and put it on a timeline from there ...
Prior to the CC, i started editing them in CS6 and I found the MOV is easier to work with but bigger in file size. And I always thought the norm was transcoding the MP4 to MOV. and I found out that the workflow of importing in GoPro Studio App, then selecting the parts that are usable and transcodes those into single clips was much easier for me. BUT if that just gives me more headache like now, I'd effin delete that and start from MP4 for now on!
Yes the footages are on a Pegasus Raid HDD linked thru Thunderbolt, from Media Browser import and arrange on timeline. The thing is, It was ok before this,now it started giving me this problems.
And, I opened a different file that also has a timeline of GoPro MOV footages it loads up fine. I closed that one and open the one with the problem then it Loads up fine on the timeline, we can see the footage in the program monitor & source monitor.
Yea, there've been a lot of changes that have been needed in workflow patterns as CC came in and has transmogrified itself. There's still some that feel that one simply SHOULD transcode into Editing Codecs because ... well, it's the Professional Way to Work. But those started when NLE's couldn't take a lot of "original" camera codecs, and the codecs themselves were often ... problematic. Testing footage both native & trans-coded, working the same effects, then exported and viewed with a fine-tooth eyeball is a good thing. And what a lot of people have found ... with decent computers, and the CC Adobe apps, the camera codec/format works as well.
Now ... there are specific things & time do do a transcode, such as bringing 4k 4:2:0 footage down to 1080 4:2:2, which with the right transcoding is a complete 4:2:2 and not "just 420 in a 422 named wrapper" as some have thought. And many others. But ... MP4 and PrPro have come a long way recently.
That whole dang thing of it's been working & just stopped, on a machine that SHOULD work ... dangnably maddening. Especially with the problem of it sometimes works.