Pond leaks

I have a medium sized pond which I completely renovated about 12 years ago, putting a new plastic sheet liner on top of the existing liner. The last few years it has obviously been leaking. Last autumn I completely emptied and cleaned it but could find no leaks. I resealed a join and refilled it, but the problem is if anything worse! Does anyone have any suggestions for finding or curing leaks. I am loathe to completely renew the liner as all the plants around it are well established but would need to be removed.