Hi,
If you have not repotted in over a year, its time. Repot them in a light mix. The north window probably doesn't give them enough light. If you can, add some supplemental light. Flourescent lights are great for AV's but they need to be about 12 inches from the top of the plant. I am adding a page on basic care to this message, see if it helps.
Fred in NJ

Some basic African violet care

1. African violets are very much like human beings. They prefer growing in similar conditions. The plants prefer temperatures between 65 and 75 degrees Fahrenheit with a relative humidity of approximately 50%. They enjoy bright indirect light as opposed to direct sunlight.
2. African violets grow well in light airy soil less mixes. The mix should be one that allows the roots to penetrate to the edge of the pot. It should not be so heavy that it retains so much water to make the mix soggy. More African violets are killed by over watering than any other method. A simple recipe that many growers use is equal parts of peat moss (or potting soil without any fertilizer), coarse vermiculite and coarse perlite. A handful of horticultural charcoal may be added to sweeten the mix if desired.
3. Light plays an important part in violet culture. Plants need between 10 and 12 hours of indirect light a day. This may be obtained either naturally or by artificial means. East or west exposures are good. Northern windows frequently are not strong enough but can be adequate if some artificial light is added to extend the light period. Southern exposures are much too hot in the summer and tend to burn leaves unless the are shaded by a sheer curtain or dappled light from an outside source. Fluorescent lights are ideal. Plants should be approximately 12 inches from the top of their crown to the light bulbs. Cool white, Gro Lux, Gro Lux WS, Vari Lux and combinations of cool and warm bulbs make plants grow beautifully
4. African violets need a constant source of food since they are potted in a soil less mixture that has little or no nutrient value. Most growers keep their plants on a constant feed system. They fertilize every time they water with a dilute mixture of fertilizer usually about half the manufacturer recommendation. The fertilizers use a three number system. The first number is for nitrogen, which promotes leaf and plant growth. Second is phosphorous which helps promote bloom and the final number is potassium which is included for general health.
5. Standard size violets should be repotted every 9 months to a year while semis and minis every 4 to 6 months. The rule of thumb for pot size is to go up a size when the crown of the plant is more than three times the size of the rim of the pot. So if the crown is 9 inches across it can go into a 3-inch pot. A large 18-inch plant can be put into a 6-inch pot. Most minis and semis do best in 2 to 2 inch pots. Since violets have shallow root systems, squat pot or azalea pots are ideal.
6. There are some diseases that can give a grower problems so it is always advisable when bringing a new plant into your home to ISOLATE the plant for at least a month before putting it with any of your other plants. Some of the problems that can occur are thrips, soil and foliar mealy bugs, cyclamen mites and powdery mildew. Each problem has its own cure from spraying to a systemic.

Thank you Fred for the tips. I actually re-potted them back in May of this year. I think you are right about the light. When I first brought them home, I had them in an east window and they did extreemly well. I moved into an apartment with only north facing windows, and that's when they stopped blooming. Moving again! this weekend into a house (yeay)I will put them in an eastern window and see how that works. I will also incorporate nutrients when I water. Hopefully I will have improvement. Will keep you posted.

Thanks again!