a climb up is an awkward movement. you have to try to bring your elbows down while you flex your biceps to keep your arms straight in order to get your upper body working properly. you really want to go up and then in, not in and then up.
it helps to have one foot higher on the wall than the other, use the higher foot to stabilize you and start pushing while you kick your other knee up to get that little bit of oomph you need to get your shoulders above the wall (thats when you come back in).
honestly, you are probably strong enough for climb ups, you just havnt used those muscles in a while (probably) so they dont quite know what to do. just focus on a good technique and the strength will come with time.