I often feel small plucks when I retrieve or see the fly line move when I am fishing flies with marabou tails but no matter how quick I strike I don’t get the fish

Trout will often hit a fly to try and stop it rather than actually take it and that’s what the plucks/movements are. Best thing you can do is keep the retrieve going or speed it up a bit until the trout really takes the fly properly and gives a good hard pull.