why is my 54 plate lwb vauxhall vivaro double cab lumpy in first and second?

hello there i needed some help with my vauxhall vivaro lwb 54plate as ive bought this van in october ,been havind problem with second gear at times,but this problem has now developed and now am havind problem with the first gear as well,as i put it on gear and press the gas pedal it feels as something is holding it back and the speed wouldent pick up but slowly it starts to pick up ,any advice would be most appreciated,thanks baz.