Once a planting media (potting soil) dries out, it can be hard to re-wet. I looked for an article from the Extension Service about this and found suggestions to submerge pots in a bucket of water for soaking, but that won’t work with your potatoes, will it?
Will a different planting mix stay moist? Some articles state that peat is particularly known for becoming hydrophobic once it is dry, meaning it actually repels water. Is a peat-free mix easier to re-wet? I didn’t find analysis about this. I did find this paper from WSU about concerns with use of peat.
This article about growing media for containers describes the possible ingredients. As you shop for next year, either pre-mixed or if mixing your own, this gives good basic information about the materials. If you hear about the wetting crystals to mix in soil, they are generally not recommended. (Read the papers under the sub-heading “Polyacrylamide Gels” here.)
After my research on this, the basic advice is to monitor your planting carefully to be sure the media is holding water. Don’t let it fully dry out because of the re-wetting challenges. Experiment with how you water. A drip system, or a low sprinkler rather than using a strong hose sprayer might work. Perhaps you need to water more often.
Monitoring moisture in the center of your grow bags is a challenge. There are moisture meters with probes, but can you judge the weight? Either by lifting or rocking it? If you water until it runs out, but the bag is still relatively light, you know you need to do additional soaking.
I hope your potato harvest next year is better.