Fermentation by itself is tricky but secondary fermentations for sparkling wines are especially so because your new yeast must fight against existing alcohol (which is toxic to yeast), depleted nutrition, and potentially toxic (to the yeast) compounds left behind as the primary yeast died out. To maximize your chances of a successful in-the-bottle fermentation you need to prepare a starter culture, let that ferment to 0 °Brix and then immediately distribute that amongst your chaptalized base wine. The “active but acclimated” yeast will then hopefully ferment the residual sugar in your closed-up bottles, creating the fizz you’re looking for.
From what you write it seems like you were directly adding yeast to finished, sweetened wine. Since that would be shocking the new yeast quite a bit, it might serve you better next time to get a dry (but still active) starter culture first, then inoculate. Even if you follow those steps, there are still many things that can delay, slow down, or even prevent a secondary fermentation from happening where and when you want it to. To help you troubleshoot the many issues, here are some possible causes as well as suggested remedies for a failure to re-ferment.
Temperature too cold: All wines like to ferment above 50 °F (10 °C) so be sure your bottles are being stored above this temperature, and preferably at least 60 °F (16 °C).
Not enough nutrition: The first fermentation might have depleted all of the minerals and nutrients in your must, especially if you started with a fruit wine like cranberry, which doesn’t carry as balanced an initial mix as grape juice or must. You might want to add a little yeast nutrient (DAP is better than nothing, but a mixed nutrient would be best) to your wine at a low dose (perhaps 1⁄4 of what you would add for primary) to make sure the new yeast have something to boost them.
High alcohol: Alcohol is actually toxic to yeast; it makes their cell walls permeable. High levels (over 15%) will almost immediately kill off many yeast strains. Re-check analysis numbers on the base wine to make sure you’re not dealing with a high-alcohol situation.
pH too low/acid too high: A pH below 3.30 can inhibit yeast growth. Check acid levels on the base wine and try deacidification with potassium carbonate or potassium bicarbonate if needed. Try to shoot for a titratable acidity below 8.0 g/L.
Too much residual sulfur dioxide: Most yeast strains are sensitive to free sulfur dioxide over 15–20 ppm. Check free sulfur dioxide (FSO2) on the finished wine to make sure too much wasn’t added.
Yeast used for secondary inoculation not the right strain: Be sure to use a “strong” fermenter recommended for secondary fermentations. Some yeast strains are very sensitive to alcohol and sulfur dioxide. The good ol’ “Prise de Mousse” and EC-1118 will work but there are other strains like “K1” that are especially good in secondary fermentation environments. Generally, any yeast advertised as working well for re-starting stuck fermentations will be alcohol- and sulfur dioxide-tolerant, but be sure to match your yeast to your base wine’s alcohol level. You could also try the new “ProElif” encapsulated yeast from Scott Laboratories, which I have heard works for sparkling wines.
Correct yeast strain but culture not active: Be sure to “proof” yeast before pitching to make sure it’s active and robust. Do not use this particular batch if you don’t see a lot of activity. Try a new packet or try a new strain.
Correct yeast strain, active culture but pitched incorrectly: When introducing new yeast into a “compromised” environment of a finished primary wine (high alcohol, possible toxic yeast by-products from first fermentation), it’s important to really acclimate the new yeast well. Try feeding the active yeast small but increasingly large bits of the primary wine, perhaps waiting around 15–20 minutes and checking for activity after each addition before inoculating every bottle in the batch.
Keep in mind that a combination of the above factors could be your fermentation-limiting problem. For example a high alcohol, low pH, and low nutrient wine will be especially difficult to get through a secondary yeast fermentation. I hope this list of factors can help you troubleshoot what could be keeping you from your ideal sparkling wine.