As a minor note, 20 deg. C (68 deg. F) is on the high side for a lager; that's more typical of ale fermenting temperatures. Lager yeasts generally do best around 13-16 deg. C (55-60 deg. F) for fermentation and around 5-7 deg. C (41-45 deg. F) for lagering. While the higher temperature probably isn't disastrous, finding some way to control the temperature better is something to consider for the future, especially with lagers. One of the reasons I generally brew only ales is because I don't have a way to keep a lager down at the appropriate temperatures (yet).
It may be a kit that says Lager on it but uses Ale yeast, e.g. Coopers European Lager, Coopers Draught etc.
Make sure the FG is both consistent over a few days and roughly as low as it should be. On my second batch I had it slow down at about 1.022 which was too high for the beer I was making, and I gave it a gentle swirl by lifting the handle and rotating back and forth carefully (to avoid splashing/oxidization). This roused the yeast a bit more and they got the job done. Most of the time I just leave things for min. 2 weeks before first checking the gravity.