I had this happen to me with a three year old batch of replenished LPD. I had been sloppy with replenishing after not using it for a while. Otherwise, it never runs out of steam.
Bromides and such do build up, and eventually it's a good idea to mix a new batch, where the new working solution is 1 part fresh stock, and 2 parts old developer. That seems to properly revitalize the activity of the developer.