Robin BP 2501h hedge trimmer?

blades lock up almost continuously after working fine for ages,but this only appears to be intermittent? It does not appear to be just because the hedge is thicker.
It happened near the end of cutting hedge afew months ago and again happened after a couple of hours of cutting.Overheating does not appear to be the cause as I opened the transmission several times to reseat blades and pack with grease.
Could it be that the blades need sharpening?