Pathetic Mint

[Marb67]

Well for the life of me I cannot understand why every new pot of mint I buy (including lemon balm out of my mum's garden) ends up looking sickly and mottled as if a small creature is eating the goodness out of the leaves. I put into good compost in a deeper pot and keep appropriately watered but the same happens every year. I used to get proper lush apple mint leaves but not so now

Again, this frustrating site gives me an "upload failed" when I try and upload a Jpeg from my ipad. File size is correct.