Hi, Christie. YES, you CAN increase your straight-leg push-ups reps! To do so, understand a few things:
Straight-leg push-ups, in addition to placing much greater demands on the motive muscles (pectorals, triceps and anterior delts) also place much greater demands on the core muscles and the hip flexors including the quads. You need to be prepared for that when you're trying to do more of these babies.
I've read many, many excellent suggestions from others here on this forum on how they made the transition and increased their reps.
Several vary the placement of the feet. Many have started with straight-leg push-ups by placing their feet wide; this creates a wider base of support which can enable the exerciser, again, to segue into the straight-leg push-up more gradually.
Several have also varied the width of the hand placement. Understand that the wider the hand placement, the greater the recruitment of the pectorals, which are much bigger muscles than the triceps. Then, as they have gotten comfortable with this and the tris and anterior delts have gotten accustomed to the new greater workload, they have gradually narrowed the hand placement to increase the workload on the tris. To increase the workload on the anterior delts, you actually want to increase the elevation of the feet; I actually prefer to do my push-ups with my feet on a 6" to 8" bench. This takes some working up to.
At the end of the day, however, is this: you can only increase your straight-leg push-ups rep count by just doing them. Start with your 8-10 set, rest at the top of the push-up position and then do one more. Just one more. Then, after a break or the next workout, do two more. Then gradually increase your push-up rep numbers one or two at a time, as you can safely. You will be surprised at how quickly you train up to them, as long as you are consistent with doing them on a weekly basis. It isn't easy, but it's more doable than you think.
When I was making the transition, it surprised my how much my own psyche was my own worst enemy. I had a number in mind and told myself (sometimes consciously, sometimes subconsciously) that that was the max I could ever do. Breaking through your mind's own glass ceiling is half the battle.
HTH -
A-Jock