Henry, to address what Rip has made reference to a couple times, a few tips:
1) (Mentally) try to shove your biceps, or even your shoulders into your ears at the top for the squeeze.
2) It looks like you might be wide on your grip - narrower may help.
3) Dr. Sullivan has an excellent video for you, with visual and sound effects and everything: Improve Your Overhead Press with this SHOCKING Cue! - YouTube