It sounds like you might have overcooked the potatoes, which can sometimes lead to uneven cooking, especially if the potatoes were large or irregularly shaped. Russet potatoes can also be starchy and become gummy if overcooked.

Here are a few tips to avoid that in the future:

1. Cut Uniformly: Try cutting the potatoes into similar-sized pieces before boiling. This helps them cook evenly.
2. Check for Doneness: Start checking for doneness a few minutes before the recommended cooking time. A fork should easily pierce the potatoes without them falling apart.
3. Immediate Cooling: Once they’re done, drain the potatoes and let them cool a bit before mashing or mixing. This can help avoid a gummy texture.
4. Microwave Cooking: If you're microwaving, make sure to add a little water and cover the dish to steam them evenly.