Synchronizers could be slipping and not lining up the gears right or fast enough. Normally, you'll start grinding pretty hard soon if this is happening. Or if you have a cable style tranny, they could have stretched or loosened. It just really depends on what you have.
It doesn't even have to be the tranny. It can be anything from the clutch plate stiffening up to the gears going bad. It could be fluids, cables, or something as simple as a little grease being off. This is why you take it to mechanic if you have no clue what to do yourself, which you have no clue since you are on the internet.
The problem is finding the right mechanic. Why? Because it could be something as simple as the clutch plate needing grease or just being replaced but the mechanic can just tell you the whole thing needs to be dropped out and rebuilt or replaced. One is a cheap fix and the other fix isn't. Since you don't know jack crap you couldn't tell. So you either find a mechanic you trust, or get second opinions from different mechanics.