300tdi Assembly problems?

Hello there i need some advice on valve stem caps? I just got a non runner disco 1995 knowing head gasket had gone set to work on the engine.
While removing head and cleaning noticed two valve caps had fallen off while tiped up so removed all for safe keeping. with head all in and push rod thought good time to pop them on. Expecting some resistance and click when i pushed them back on but nothing. ok there old maybe worn so orderd new exactley the same. They just sit on top worried they will just fall off.

Just be careful until you have the tappets adjusted properly. They wont come off once eveything is back together and adjusted