Edit: If cooking longer softens the potatoes, then this isn't what's happening. In that case, well, you just need to cook longer. The main variable is probably temperature (maybe the pot isn't actually all hot for all 30-45 minutes), followed by variations in cut size and in the firmness of the original potatoes. But the rest could apply to some readers too!
I'm going to take a wild stab at this. It can explain this sort of thing, but it does depend on how you cook your stew. The following is a quote from On Food and Cooking (excellent book!), one of my favorite discoveries from reading through the sections on fruit and vegetables.
It turns out that in certain vegetables and fruits - including potatoes, sweet potatoes, beets, carrots, beans, cauliflower, tomatoes, cherries, apples - the usual softening during cooking can be reduced by a low-temperature precooking step. If preheated to 130-140F/55-60C for 20-30 minutes, these foods develop a persistent firmness that survives prolonged final cooking. ... Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 50C (and inactivated above 70C), and alters the cell-wall pectins so that they're more easily cross-linked by calcium ions. At the same time, calcium ions are being released as the cell contents leak through damaged membranes, and they cross-link the pectin so that it will be much more resistant to removal or breakdown at boiling temperatures.
So, if you started the stew from cold (or drastically reduced the temperature by dumping in the sweet potatoes and other things) and had it on low heat, so that it took a while to come up to temperature, then your sweet potatoes may have been in that 50-70C range where the enzyme is active for a while. So maybe while your length of cooking time is consistent, your time before simmering isn't; that could depend on how cold the ingredients were before adding, the pot you used, whether you got exactly the same setting on your stove, the temperature in your home, and so on. (I know you said this isn't it, but having larger chunks of sweet potatoes could conceivably contribute to that too; the inside would take longer to heat up, so you could end up with a firm interior, and the softer part on the outside could get rubbed off.)
Sweet potatoes do also vary some in firmness, but certainly not by that much. It could still be a factor, though, in combination with the rest! In any case, if you want to avoid the firm potatoes, I'd try making sure to get everything heated up quickly.