The last few times I've made it to 20/30/40/50 ish my timing and ability to rotate the strike (fairly poor anyway) seems to go from reasonable non existent and i end up stalling the innings and getting out wasting plenty of starts and balls in the process.
This can happen if you haven't had the strike for a while or if the ball has changed condition, or the bowlers have worked you out a little and got a plan to contain you - it may not be your fault.
When it happens, it is just a matter of changing you game plan - you aren't middling the ball, ask yourself so what are the bowlers doing differently? think where the scoring areas are off a bowler and be patient.
you may find just moving your stance, either further forward or further backwards can help - or depending on your experience moving more to the off-side to force the bowler to change their line.
Fitness has been mentioned and may, or may not be an issue - the key is don't panic.