I found temperature can be a little off putting, I started with a stick on temperature strip, at 16ðC no activity but at 18ðC it was going fine, however as time went on it seemed to stop before it was complete, as I moved to using a thermostat with digital read out I realised that the strip is showing a figure some where between ambient and the brew, where the sensor held onto the side of the fermenter was showing closer to real brew temperature, so at start the stick on strip shows under the read temperature, but as the fermenting starts to slack off, the two become closer in what they say.
This showed me the error of reading the air temperature, you really need to measure the fermenter temperature. At 18ðC with the kits I use your just on the edge, I set at 19ðC +/- 0.5ðC and if your sensor is not measuring the actual fermenter temperature it could be it's just a little too cool. I hold it at 19ðC for around 7 ~ 10 days, then the next 7 ~ 10 days I lift the temperature to 22ðC it lifted as I have read it needs to be a little warmer to finish off, and also if it has not completed then you can end up with too much pressure in the bottles.
I still use plastic bottles just in case, I can test pressure without opening, plus release a little if needed.
I have experimented with a electronic thermometer, both comparing with the thermostat and checking the difference measuring at different points. Under a sponge pressed hard against the fermenter compared with in the centre of the brew there was just 0.5ðC between them, however measuring air in the freezer compared with side of fermenter I recorded 8ðC in air and 19ðC on fermenter side, however against what I expected the brew did not even dip 0.5ðC below set temperature, I had started it around 2ðC too warm. Maybe because I am using a frost free freezer which has as with all frost free a circulating fan.
On heat then it was a different thing altogether, on heating one has to be very careful not to use a heater which is too big, I found 8W was ample, above that easy to get a hysteresis temperature wave as it over heats due to either the heater being too big or having too much bulk.
Once I have done a kit a few times I find the times are quite steady if it takes 15 days one time as a set temperature it will also take 15 days next time, however until you have worked out the time, it is down to using the hydrometer, I still do a final check with hydrometer, but don't bother until the end now.
So I would question where the temperature sensor is, and if your not just a little too cool.