I have just purchased a 1990 toyota pickup truck. Today the transmission would not shift out of low into second for a while and then it did. It is an automatic transmission. It has been shifting fine ever since. It did this briefly day before yesterday.

What could be causing the trannie to not shift and then to shift OK.

I'd drop, inspect, and then clean the transmission pan. Change the filter and refill with the proper transmission fluid. To do a really good job, read online about how to change the fluid in the torque converter. Change fluid and filter every 24,000 miles or 2 years, whichever comes first.

As stated above, maintenance is the first priority here.
Also, be sure the downshift cable is not sticking within the sheathing. Just give it a tug at the throttle and see if it springs back.