Well, it seems that UEFA themselves agree.

It turns out that the fine of £13,000 was set by a "semi-independent panel", and UEFA appear to have decided that that fine may be insufficient. The amount appeared not to take account of PSV's poor record of racism and violence. Nor of their apparent refusal to accept that they have a problem (the story from their official spokesman was that it was a very minor problem limited to a handful of fans, who they identified and banned)

One reason given for the low amount in the initial verdict was Arsenal's failure to make an official complaint. But it looks like UEFA, quite rightly, are deeming that to be a poor reason for failing to punish racism. Instead of criticising Arsenal, they are now asking the club to provide evidence. "Statements to the media are not enough, " they say, with the implication (we think) that if Arsenal players like Henry and Cole say the same things to UEFA which they've already said to the press, then PSV may well find themselves in deeper water than they'd thought.

A larger fine may be in the offing, and the possibility remains that PSV fans will be banned from a Champs League match. See eg Sky Sports.