。。
She's not lacking in compassion at all. She's just shy.
Literal
She [topic-は] at-all compassion [in-に] lacking is-not. Just shy only is.
少しも~ない ('not at all, not in the slightest') is an emphatic negation pattern. 思いやり (おもいやり, 'compassion, consideration for others') is a valued virtue in Japanese culture. The second sentence ただ~だけだ ('it's merely that') reframes the situation: her apparent coldness is really just shyness, not a lack of caring.