I'm not so much talking here about TV series that went bad or hung on too long for their own good ~ although that might be interesting to talk about too ~ I'm more interested in TV series that were still good when you lost touch with them or ran out of steam.
I grew weary of Mad Men's glacial pace by series 5 and had at least a year off before watching series 6 and 7. I think it may have sagged a bit in series 4-6 [and even the first half of series 7] but it certainly ended very well and I'm glad I returned to it.
I watched and thoroughly enjoyed the first three series of Game Of Thrones and then found, strangely, that I had no stomach for series 4. Had the novelty just worn off? And yet it still seemed really compelling at the end of series 3. I have series 4 and 5 on my hard disk and no doubt I will get back to it one day.