manual transmission car will sometimes go into reverse and not.?

try double clutching when u put it in reverse mine also does that from time to time all i do is double clutch it and goes in gear just fine

First of all Reverse does not have a syncronizer. My guess would be that the clutch is sticking slightly so try this. Before putting it in reverse,with the clutch fully depressed, put it in 2nd gear, this freezes the gear assembly. Then try reverse right away. If this is the answer you are looking at a probable clutch replacement.

This sounds like worn linkage. Sorry but I don't know if your transmission has external linkage or not but either way, there are likely some adjustments that can be made. It would not be a job an amateur could do, so you should let a dealer mechanic or a transmission shop look at it.

The cost of diagnosis is not the same as the cost of repair. Having it diagnosed is not expensive.

Sounds to me like the clutch is dragging an is not fully disengaging. This is due to wear on the parts over time, and a simple adjustment is all you probably need.

Good Luck.