Something in the portfolio is always doing badly

That is not a failure of the portfolio. It is the definition of the portfolio, and it is the reason clients find it so hard to hold.

The structural discomfort

A diversified portfolio holds things that behave differently. So in any period, some of it is doing well and some is not.

The client sees the part that is not, every time they look, forever.

Why it feels like a mistake

Because it is always possible to identify, afterwards, what should have been held instead. The comparison is available continuously and it is always unflattering to something.

A concentrated portfolio does not have this problem. It has a different one, which only appears occasionally.

Diversification guarantees you will always regret part of it. That is the price of not being exposed to being wrong about one thing.

The uncomfortable years

Long stretches where the diversifying assets are a drag are normal and can persist for years. That is long enough for a client to conclude the approach is broken.

Preparing them for that in advance is more effective than explaining it while it is happening.

What to say

Be direct: this portfolio is built so that no single thing being wrong is catastrophic, and the cost is that something in it will always be disappointing.

Stated at the outset that is a design principle. Stated afterwards it sounds like an excuse.

What diversification does not do

It does not prevent loss, and correlations between assets tend to rise in exactly the conditions where separation is most wanted.

Overselling it as protection sets up a specific disappointment at the worst possible time.

Where the honest limit is

Diversification manages the risk of being wrong about a particular thing. It does not manage the risk of the whole market falling, and no allocation does.