Since you have got low frequency gain of 200 during AC analysis ,that means that the common mode voltages are proper and circuit is working fine .If you are getting slope of only 20 during DC Sweep ,that indicates couple of things
1)You are measuring slope in Input common mode range .(First calculate this theoritically and given extreme limits of ICMR and do a .op analysis to check all transistor's are in saturation .Then measure the slope in ICMR range .
2)Some times you have to invert the slope(Depends on which i/p you are sweeping) and this gives you the DC Gain .
3)Last probability could be your ICMR range is too small and you are checking the slope some outside limits of ICMR .
Hope this Helps !!!