You may want to do several things.

1) Use an oscilloscope to check that the servo timing is what you think it is supposed to be. You may have already done this step but I comment because all too often we think we fully understand all the characteristics of the timer, prescaler, and mode control registers only to find out that what we coded up does not produce the timing we think it should be.

2) Be aware that the typical pulse repeat time for most servos in the industry is supposed to be in the 20 to 30 millisecond range. However there may very well be servos that require pulse repeat times faster or slower than this range. Most web pages on the subject keep repeating that the acceptable range for any servo is manufacturer dependant. You may have some that just do not work at 20 millisecond range.

3) I think you definitely want to look at the pull-up issue on your board to see if you would need to add one. Most 8051's that have bog standard I/O pins do a pretty poor job of giving you a good rise time when there is any type of load on the output. If the pulse you are generating has a lazy rise time it could appear to the servo to be narrower than the wave you thought you were generating. Clearly an oscilloscope will be required to verify rise time of the signal and is the only way to be fully certain. You couldn't go wrong with adding a 2.2K or 3.3K pullup resistor on the MCU output pin.

Michael Karas