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You are probably doing simulations wrong. If you're doing AC analysis, apply inputs of +0.5 and -0.5. If you are applying 0.5 to both inputs, then you're simulating the common-mode gain which should increase with frequency in most cases.
Plot the AC output, this will be your AC differential mode gain.
Also your inputs need to have a DC bias since your lower rail is ground. Let the DC value be Vdd/2 for now. It can be optimized later. If your inputs have a DC of 0 right now, transistors will not be working properly and won't act correctly as an amplifier.
First, you should assure that every devices in your circuit working in proper condition, USE DC and OP to debug your ckt. after this, do ac analysis, also ensure devices' working conditions are the same as in DC analysis. Simulator or commands syntax are not the chief criminal, the KEY is your understanding into the ckt, good luck!
i assured the devices work in saturation region in OP analysis but working conditions changed in AC and surely this caused inproper working. how to polarize inputs during AC simulation to have DC offset. this way i did doesn't work.