Matlab only plots 2 points instead of time series

Just recently, matlab began plotting my time series for only 1 dataset as a line connecting the first and last point instead of every X minutes. I made a script to make the file it calls smaller, and that worked! For a couple weeks. It is now doing the same thing again.

I have this same script for 100+ data files, and it only does it for this one site. The best way I can explain this is in a graph below.

The data axis is fine for the 1st three, then upload the file in question and the dates go to line number instead of date. Like I said before, I cut the file I retrieve this data from in half and it worked for a while, then it started doing this again.

Has anyone had this happen before? Thank you.

[dpb]

I'd guess there's something wrong with the date input for the offending file. Isolate where the actual point is that causes it to fail--you know it worked for some smaller set; increase that size until it fails; bisection from that point to the end would narrow it down pretty quickly.

Once you've found that location, go look at the input carefully there and I'd expect to find something wrong...perhaps a date came at an even point and is missing seconds or fractional seconds that are in the input conversion or an AM/PM is missing or the like.