Yellowing Pinks

Hi All,

I got some pinks early in the year as plugs, and they've grown pretty well. They're living in pots on my balcony and most of them put on quite a nice display this year, which pleased me, being their first year.

Some of them have maintained a nice tight mat structure as they've grown, but others have become rather untidy, with several healthy looking clumps hanging loosely apart from one another and then a brownish-yellowish centre with rather dried out leaves. 

I'm not sure what to do about it, if anything. They don't look very pretty as they are. Should I maybe divide them into smaller, healthier clumps in spring? I may have over-fed them.

Thank you for any advice!