I planted a clematis in March while it was dormant. When doing so, the
(thin anyway) old growth broke off. I saw some growth about a month
ago, but the biggest of the emerging stems got stuck at 5cm. I
tried to examine it and I realised that it was munched off just under
the soil surface. The same thing happened to another 3 of them, that
did not even make it to 5cm.

When I planted the clematis, I sprinkled some slug repellent (the bio
one, that just deters them from crawling to the target) and topped it
up again another couple of times since then. The brick I am using to
provide some shade to the roots has been home to 5 slugs in the past
(but I am not checking it all that regularly). Any idea what's eating
the clematis?

Thanks in advance,
Kostas