When you say the buttons 'don't work', do you mean (1) nothing happens at all, or (2) they don't work as intended, i.e. there is some unwanted change?
One cause of the problem may be switch contact bounce. I don't see anything in your schematic to overcome this. Switch bounce will cause a series of pulses to be generated instead of a single pulse, giving an erratic change in the count which, because the 7490 is a ripple counter and depending on the switch characteristics, may only occur at certain count values.
Another cause may be interference on pin 2 (with pin 3 already high), resulting in premature resetting of the 7490.