There are any number of ideas on the Forum about stuck ferments. In no especial order they include:
The temperature is too low. In winter the indoor temperatures can drop so low the yeast "goes to sleep" becoming inactive and requires a significant rise in temperature to restart. Insulation can protect the FV from temperature variations and there a number of ways of adding heat using thermal pads if it not feasible to move the FV to a suitable location during the coldest time of the year.
The amount of yeast pitched is inadequate. My understanding is that the viable yeast cells in the packet will tend to reproduce until the available oxygen is used up. This makes aerating the wort very important - e.g. by pouring the water into the kit mixture from a fair height - a couple of feet dropfrom a seies of jugs or similar.
If the pack of yeast is old or has been badly stored, the number of viable cells may also be too low.
There is insufficient yeast nutrient. Not sure on this one as Wherry is a 2 can kit and the most notorious sticker. A good source of nutrients for beer yeast is malt.
Unreliable Hydrometer readings. A sample needs to be shaken vigorously before taking a reading to get excess gas out of the beer. Otherwise bubbles quickly form on the body of the hydrometer and push it up, giving an artificially high reading. Wait until the foaming subsides so the measurement can be taken properly. At 20C water should read 1.000 on a hydrometer, so test this before you go believing anything it is telling you.
Impatience. Four weeks in the FV is unlikely to harm the beer. If it sticks, aim to make sure that the temperature is brought up over 20C and try adding more sugar. Not sure why this works exectly, but it may be analagous to lighting a coal fire using newsapaper and small sticks. Giving a gentle stir brings more yeast cells back into contact with the beer if they have become stuck at the bottom, but introducing oxygen at this stage is a really poor idea.
All these are discussed at some length and with some frequency on the Forum
