Almost everything that can go wrong gradually in a battery looks like a loss of capacity because that's the primary symptom people notice in most applications. You have to be able to monitor individual cell voltages through a charge/discharge cycle to glean the actual mechanism responsible for the loss in usable capacity. In a Li ion system, the easiest thing to fix is pack imbalance. To reset the pack, bring all cells to 100% SOC, individually if necessary. Use a cell log or similar to record the next discharge cycle to verify capacity is reasonably even across the pack after the rebalance.
The other culprits to watch out for are actual capacity loss, and impedance growth. They tend to go together, though not necessarily in predictable proportions. If simple capacity loss is the dominant issue, voltage sag on the affected cell will not stand out as worse than the other cells in the pack, but it will hit LVC earlier (possibly much earlier by the time you notice) than the other cells. If impedance rise is the dominant issue, then the cell will also have greater apparent sag because that "weak" cell has to put out as much current as all the other cells, but the added impedance means greater power lost across the cell and a corresponding drop in voltage. This can also be seen as a decrease in the cell's "discharge efficiency", meaning the cell may appear to have lost capacity even if it has not in reality. It just burns up more of its capacity across itself as it discharges so less power is available to the intended load (Peukert losses). It will charge slower for the same reason, meaning a need for more regular balancing, and longer balancing times as the pack continues to age. This also means the cell will run warmer which is generally bad for the health of any cell, so once impedance growth begins it tends to compound itself, especially if you are running the cells near the limits of their capabilities. In reality, true capacity loss and impedance rise tend to come together in some proportion, so real life examples of only one or the other occurring in isolation are quite rare. There are other ways to discern one from the other, but they are generally of little use to most end users who just want their battery to work normal again, which at some point will mean cell removal or replacement, if not replacement of the whole pack.
